I feel for you.  My DH84 let SD60 move in here about 5 years ago without discussing it with me.  She was here for a 10-month nightmare of drug use, lying, manipulation and theft.  My DH thinks he can handle any situation but he's never been able to say no to her.  

I started having physical symptoms from the stress of living with a lying druggie so after my ER visit, we ( mostly I) decided to subsidize her living elsewhere.  She's on disability.  You didn't say whether your SS works.  We separated our finances and I told him how much we'd contribute and not a penny more.  If he ever moved her back, I'd leave him.

I feel for you.  You know it wont work.  Feeling empathy for you.

What the hell? He didn't even consult with you? I'd start planning to move out. 

Be ready for CPS regardless.  I'm sure the child's mom will be calling in on this SS and I would be too.  I'm lenient on pot, but no way on harder drugs or pot heads who are non functional degenerates. Your husband the enabler should be left with this shit storm.  
Get your ducks in a row before you find yourself interrogated by social workers.  No way I'd be cleaning up and taking care of this child or SS.  All on your DH. 

So your H is undermining SS's partner, enabling SS, and disrespecting you by treating you as if you don't even exist or have a say in your own home.  That wouldn't fly with me.  I'd be moving out (or kicking H out, depending on the housing/financial situation) unless he reversed course.  He made the *wrong* choice.

It's best to start out as you mean to go on. Don't cook, clean, wash, shop or babysit for your H's son. Don't be guilted into caring for his child in any way or you'll find yourself doing ALL the work. Don't make it easy for him to be there, and make sure your H is as uncomfortable and inconvenienced as possible.

Tell your H you'll give his adult son thirty days, and then he's OUT. Then, deal directly with your SS. You know your H has no spine, so cut him off at the knees and take control. Be the B you need to be to keep their crazy out of your home. What's key is to treat SS like a temporary roommate, not a guest. Give him a tour of the house. Show him the laundry room. Show him where he can keep HIS soap, HIS hangers etc. Show him the kitchen. Point out HIS space in the fridge and pantry, show him where the dishes and silverware are, and make sure he knows how to run the dishwasher. Tell him in no uncertain terms that HE is responsible for taking care of himself and his child, and any other house rule such as quiet time after ___ X o'clock. And make sure he knows he must be OUT by 5 pm on X day. Hang a big calendar in the kitchen, and mark off each day in red.

Be firm, be direct, and make him feel uncomfortable as Hell. Many of us have wasted time pretending and being nice, and for what? You DON'T like him, you DON'T want him in your home, and you're going to make his life Hell unless he finds someone else to use.

Exjuliemccoy has given you the BEST advice! Please follow it!

Your hubby isn't going to like it, he'll accuse you of hating his son, hating his granddaughter, hating him, so be it. Tell DH the truth! You don't want drugs in your home, You don't want to be a babysitter, You don't want his son living with you.

Tell DH that you don't like his son, that he's not a good father or a good person. Don't hide from conflict, let it all out and make sure DH knows exactly where you stand. It will be for best for your mental health, your marriage and your life.

Read and re-read exjuliemccoy's advice and follow it to the letter.

I saw it that you have other children. Do you realize what is at risk if CPS gets a whiff of his drugs? Protect your own! Get him out NOW! 

I would be livid.  That is not the kind of decision that one partner makes without speaking to their spouse!

The ONLY thing I might allow in this situation is that when the guy has custody he can "stay over" with the grandbaby (and again as mentioned above.. you do nothing to aid). BUT.. I would require him to live elsewhere... he doesn't get to live in your home.. If it is for the sake of the baby.. he can visit and stay over when he has custody.. bet it won't be much.

Nope. 

It's not like your husband is planning a poker game on Saturday night.

he swears if he continues with drugs & neglects his baby he's out

Does your husband seriously think that his son is going to quit smoking pot and doing drugs? He has lost his license because of drugs and has chosen drugs and gambling over his child and the mother of his child.

Now, he's going to have your husband enabling him. He has no reason to change or grow up. He probably doesn't care if he sees his baby or not, but she will sure be a great tool to manipulate your husband with.

The child's mother can make sure she's safe, that's not your job. You two will just end up caring for her all the time.  If it's really about the child, then make a deal with the child's mother to have the baby every other weekend and allow your SS to visit her then at your home. No need for him to live there. 
 

This is about your husband enabling his son's poor behavior and thinking he can rescue him. 

I agree with tog. Allolwing him to live there so the baby is safe when he has her is just an excuse and not really logical thinking. Besides, your husband has no idea how often, if ever, his son will actually have the baby, especially, considering your SS's priorities. Hopefully, the mother will see to it to limit visitation if she feels the baby would not be safe.

Is this your husband's answer for the next 17-18 years? "Doing it for the granddaughter"?