from your post, i feel your dh is an enabler. And in my opinion enabler's don't change.

I'm a person that could've ended up in this exact scenario...as the "skid," not the parent. I had a very normal, average upbringing, which is probably what saved me. I struggle to function normally and to raise my two kids alone. Too much stress can be debilitating to me. And being a single parent is stressful. It sounds like he has coping issues. There is likely something wrong that counseling can help. He is likely self-medicating. There are no easy answers here, but realize there are people out there struggling day to day that you might not understand. And he probably doesn't understand it to explain it to anyone.

If he's making threatening statements like he's going to hurt himself or others, you can have him detained my the mental health professionals in your area. I'd say lay down some firm rules such as he has to start counseling and that you and DH get to go to the first or second session to explain your concerns.
If it's your DS and DD why are you allowing this seemingly disturbed person to spend time with them? He can't possibly be a good influence and you must be beyond worried that if he's possibly using drugs, he's influencing them to do the same. There should be a requirement that if he's living at home with DH and you, he needs to start therapy and/or work, especially if your DH has a job waiting for him.
Tough love. Do you have anywhere YOU can go for a while? Friend, family? Perhaps your DH needs to experience you being gone for a bit to get the picture and have a wake up call. Your DH is a total enabler as a previous poster said but I do think enablers can learn new tricks.

The "kid" has had plenty of chances and time to straighten up. Neither he nor his father is going to change and you could go to your grave with him in the guest room.

Tell your husband you want a legal separation and intend to live apart until the kid moves out never to be allowed back. Enforce your request by making temporary arrangement to live elsewhere.

A legal separation is a financial divorce. You remain married but all your assets, income and obligations are separated. If you have some assets in common, like a home, they can be split via undivided interest and one or the other allowed exclusive use and responsibility for - while ownership remains in both names.

This way your assets are protected from something stupid your husband does by loaning his uninsured son his/your car and he kills a Doctor who just got his diploma and license, has a wife and 4 kids who sues for your home, cars, money, business and all future income.

Just as importantly its a major first step which will either get his attention and he'll toss the son or he won't in which case you'll have a clearer idea of what to do next.

You don't have to go through with the legal separation but do move out. The point is to see what he does and go from there.

As you are at a point where you are considering divorce, maybe its worth saying 1 month or whatever you decide and say I want him to leave or I will/dh will, this man needs some impetus to pull his socks up, your Dh is giving him an easy ride, why would he want to make any effort, no doubt he dos'nt contribute with chores either? Its your home too and you have a say.

I would go as far as telling your DH that you want out of your marriage if this continues. That living like you are with him there is deal breaker for you. That he has to get on board with getting SS out asap. Try suggesting that you give SS 30 days to find a job. Maybe offer to pay the deposit and first months rent on a place for him and check out goodwill/craigslist for some cheap furniture.

Give him the tools and then let him sink or swim. He is an adult and does have to get it. You have dealt with this for far to long and your H is just enabling his behavior. You have to put an end date on this now.

I guess I'm not as tough as the others posting here. If he was medically discharged from the army, you might want to help him get started on applying for disability based on mental illness. It's a long process and he will need a lot of help with it, but eventually, when he gets it, it should be enough to cover housing and health. I would try to get him hooked up with a psychiatrist rather than a therapist, because I imagine that he could use some medication, and, in fact, might have to be on medication in order to qualify. I would also get copies of his army records. This may be more than you want to do, but it is an effort to get him well and self sufficient. Mentally ill people need help their whole lives and he may be turning to his dad as his last resort. Give him this kind of chance and he may flourish.

Usually, as long as insurance is on the vehicle by the owner, anyone can occasionally drive the vehicle with the owner's permission and that driver is covered. You can doublecheck with your agent but I know in my state that is a fact.

Given this history of mental illness, it's a lot harder to give advice. No doubt this person has problems to be in a situation of being adopted at age 8, no telling what kind of family loss he suffered in early years, then he got adopted, then that family split up and by moving away he basically lost his father.

You might consider talking to a mental health professional to see what, if any, kind of treatment would actually benefit SS. I would think mental illness would require some kind of outside intervention, that the person might not be able to find it within themselves to go after the help they need.

Separation, as already mentioned, might be a good answer for you but just make sure it works in your state. In my state there is no such thing as legal separation. Income and assets could be split but as for liability and obligation, that would be difficult to split.

It would be difficult to enjoy being at home with this hanging over your head but your DH sounds like a decent person who is trying to do the right thing. I do know it is quite terrifying to feel that someone you love might kill himself. At some point your husband has to recognize that this is no way to live, for his son, for himself, for his wife, for the other two kids, and take real steps to help the son get well. Or at least get better.

This sounds more like a case of mental health issues than anything. DH needs to take SS to a DR. ASAP. He may have PSTD or severe depression. If he does have either, there is nothing your DH can do to "change' his behaviour. If this can be either diagnosed or ruled out then a plan can be put in place. I hope DH looks into this right away and maybe you can have your home back sooner rather than later. Wish I had better advice but this is my take on it.

You mention being age 61 as a reason for not wanting to do anything too drastic at your age. But by the same token, if you let things drift, you will see an increasingly helpless old age with this guy in the property battening off or neglecting, or worse, emotionally abusing you. Precisely because of your mature age, it is time to act not just to react -- before it is too late and you are trapped with the guy as you become increasingly disabled yourself.