Calculate the cost now (save a bill for proof) and whenever he gets insurance show your DH the difference and tell him that's how much extra he needs to pay to cover SS.

I don't think there is any way but to get your own policy.  Considering worst case scenario, any discount you get now won't matter if SS kills someone.  Not sure what state you are in but the best thing to do is talk to your insurance agent. They've heard it all and they are the best qualified to answer your question.  
 

My youngest is starting drivers training tomorrow.  We now drive cheap used cars just to keep our insurance affordable.  This is a touchy subject in Step world.   IMO the most important thing for a step parent is to protect themselves from semen demons like yours.  This is a hill to die on.  

Will your still increase if SS gets his own insurance for his own car? Our agent recommended titling the car in son's name and getting his own insurance.

Note: because he was 18 our insurance company would not insure him on his own. He ended up getting the insurance through the state risk pool. It was more expensive for him than being on our policy, but our cost did not rise (son paid).

Why does he have to go on your insurance? He's 18. If he gets a car, it's in his name and he gets a separate policy. That's how I handled this issue with my bio kids. When they turned 18, the car titles were transferred to their names with their own policy. Of course, both my kids worked even while in college. And we did help them. But by doing it this way, we had zero liability for any "kid car" issues.
 

Yes, his rates will be higher, but worth the price to be free of liability with the car. 

In my state, if we added a stepchild to our policy by adding a car to our policy... then the rates for "all" our cars would go up.  Their logic was theoretically all drivers in that household have access to that car.

What ended up happening is that SD was actually living with her grandparents at the time.. so my DH got a policy for her and him at the grandparent's house on that one car.. vs adding hers to ours at our address.  different company too.  She was shy of 18 when that happened.. at 18 she transitioned to a policy without dad on it.

I'm curious though... your SS is 18.. what do other 18 year old's do regarding ins in your state?  Are they not legally able to get a policy in their own name?  Or.. is it that your DH wants to give access to your vehicles?.. and just add his son to the policy as a driver.? 

If he is added to your policy as a licensed driver.. I'm guessing the assumption is that he "could" drive any car you have on that policy.

The way to potentially fix this is that at 18.. he has his own car and policy separate.. as an adult.. and is not "added" to your personal policy. 

This might be easiest done via one of those companies like geico or progressive.... Ideally, if he is driving, he will have his own vehicle.. vs be borrowing and potentially wrecking your and your husband's. 

The reason why the cost jumps.. is a newly minted driver has access to any and all the cars in your home.. and that increases risk of an accident.  

Personally.. the best way is to have the kid have a car in HIS NAME ONLY.. and an insurance policy in HIS NAME ONLY.. that limits liability if he is not driving your cars under your policy.. 

if it comes to the point where your DH insists he is added to your policy.. simply calc the difference and make your SS or DH pay that.

I agree with your basic premise that an 18 yo SS needs that job first and should have a desire to drive.. I mean.. what is his plan for "life?"

I'm going to add that the insurance premium is not the big issue you need to worry about.  If you or your husband own a vehicle that your SS drives and he causes some huge accident.. kills someone.. causes damage that exceeds policy limits.. one or both of you could be on the hook for that bill.

For that reason, I advocate putting car soley in SS's name and insurance in his name only.. to limit the blow back liability on your household.

... or you may be in for a nasty surprise.

Depending on where you live and what type of policy you have, maintaining a policy separate from your SS may not be enough. Most insurance companies take all licensed drivers within your household into consideration when they set your rates, even if those other folks aren't covered by your policy. In other words, if SS gets a license, your rates may go up even if you have a separate policy and do not allow him to touch your car.

If you have a separate policy and can show that your SS has his own vehicle (with his name on the title) and is covered by a policy that meets your state's minimum requirements, you MAY be able to get the extra cost removed from your policy. If you do that, and your SS later "borrows" your car and gets into an accident, you will likely have to eat the entire cost of any damages, including civil judgements. 

Your best bet is to talk to your insurance company/agent before SS gets his license. This is a common situation and you have options. Get everything arranged to your best advantage before the DMV gives SS his license. Otherwise he could end up costing you a bundle. 

Something everyone should know but almost no one does: All insurance companies do regular checks with the DMV for the status of all licenses matched to a policy holder's address. If you have someone in your household who becomes newly licensed, gets a DUI, has their license restricted, suspended, or revoked, or who has any other negatives added to their driving record, your company will find out. The company can then potentially raise your rates, change the terms of your policy, or cancel it entirely, even if the person in question isn't covered by your policy. This applies to everyone who lists your residence as their address with the DMV, including that friend who lived in your basement for a few months ten years ago and never bothered to update things when he finally moved out. 

He joins the military and you go empty nester status. He can license, insure, and provide for himself.

That is what we did with ours when he turned 18.  We did not facilitate his DL until he was 18 and had commited to the delayed entry program in the military.  A few months before he left for basic training we bought him a brand new car and abandoned him at my parents to get his DL.  His brand new car sat in their driveway until he finished driving school, he went through my dad's driving bootcamp, and he got his license.  After about a month he arrived in our driveway.

Make it easy on yourselves.  This is an 18yo not a young child. He can figure out how to get a license, a car, and insure it for himself.

I do not put up with insurance companies extorting money out of me for coverage that i did not initiate.  They can F-off.  I have instantly fired insurance companies and demanded the refund of the premiums for remaining annual coverage for that crap.  Most recently, we sold a home.  Someone tried to steal title of the home by initiating credit card applications from our address. As soon as those actions were initiated our auto insurance immediately added this random POS to our policy and upped our premium. I went ape-shit on their idiot asses.  The actual insurance company was a PITA about it, so I called out insurnace agent's office and spoke with the admin we had been interfacing with for the better part of 20yrs. I explained the situation. Her exact response was "Oh hell no!" and then she told me she would call me back.  My phone rang in about 10mins with her call back.  Before that call ended the insurance company had texted me informing me that the scam artist had been removed, my premium had been adjusted accordingly and included an appology. I had told them the next conversation would be with my attorney and that I was already filing a complaint with the the State department of insurance.  I held an all lines insurance adjuster license for two decades and my complaint included my license number.  I do not mess with bullshit like this other than to lay waste to it.

It is none of the insurance company's business who is in your home or why they are in your home.  They can make up bullshit all day long about everyone in the home having access to the vehicles, etc... So F'n what?  It is still not the insurance company's business who you insure or do not insure on your auto policy. If an uninsured driver "steals"  your car you are covered under the comprehensive coverage element of your insurnace. Of course the "thief" will face charges and is liable for any damage their actions cause.  Which isn't your problem. Just never give give permission for an uninsured driver to use your vehicles.  

IMHO of course.

Good luck with this.

And keep them in a safe place. Not in the home.  What you pay this year and $. You subtract the $ of new insurance policies ​​​​​​   
'and DH pays the difference.   Make copy's of new policy ect 

DH and I are on the same car insurance. We also have our home insurance bundled. SD15 hasn't even taken driver's ed yet and doesn't have a permit. I am also saying she needs a JOB before getting any of these things. We have grocery stores and coffee shops within walking distance or a short drive by DH to drop off/pick up when SD works. 

I am hoping DH keeps putting off SD15 taking driver's ed. I will definitely NOT be staying on insurance with DH if he wants to add her to the policy. Technically the policy is in my name. I would have DH drop and get his own with SD15. I would keep mine bundled with the house. 

HOPEFULLY, BM will add SD15 on her insurance. I have heard that as long as you provide proof of insurance from another company, your premium shouldn't increase. Also make sure that SD does not register our address on her driver's license. She can use BM's residence.