I am going to be open and honest with you.........I hope you can understand that your stepson is a adult, but has been treated like a child, therefor he has no desire to become a fully functional adult.
Often when we continue to try and solve our children's problems we are only enabling them. He is over the age of 18 and should be taking more responsibility for his life. If he wants to take the trip, and get into trouble you can not stop him.......neither can his father. Though that doesn't mean you have to accept it and pay for his mistakes either.
My question is this.....if he is not working, how is he paying for gas for his car? What about insurance, his phone bills, etc? I am assuming you and his father are paying for these. While it is acceptable to help him out with food, and necessity's if he is in in school you are not required to pay for these extra things if he is not making good choices.
It sounds like he isn't in any danger of you damaging him.........it sounds like he needs a wake up call. What happened with his mother was a sad event. And yes this could be causing some of the issues. But allowing the issues to continue and treating him with kid gloves simply gives him a excuse to act this way.
The fact is many people have lost their parents......some children at much younger ages. While this was painful for him, it is not a reason for his actions. If you and his father refuse to deal with his actions based on his mothers death, when will he take responsibility? Do you think the police and courts are going to accept that his actions are based on the death of his mother when he was 13? The truth is they will not......what happen was horrible for a young child. But not a reason for his actions.
What he is doing, he is doing for himself....if you allow it based on his childhood then he will grab a hold of that excuse and hold on for dear life. The issue is......someday he is going to get into real trouble and you will not be able to bail him out.
Now is the time to rein in his behavior. Time to sit down with dad and let him know that things have gotten out of hand. While you and have father have no real control of his sexual behavior and should not.......since he is a adult. You do have control over some of the luxury's he has in life. If he is not taking care of school then maybe it is time to pull back on some of your funding. Refuse to pay for insurance, gas, cell phones and other expenses. If his collage is near you, then maybe it is time to tell him you will no longer be paying for a apartment or campus housing. Give him the choice to move back home and go to school, or get a job and pay for it himself. If you think he will change then give him a set time. Say next report card he must have brought his grades up to a set amount or funding for the apartment is gone.
If you simply want no part of it........then sit down with dad and let him know. Explain that you love the boy, but his actions are not only a danger to himself but are causing you conflict and pain. Let him know that from this point on he will need to deal with his sons actions. Explain to your stepson, that you love him but his actions are hurting you so for your own personal wellbeing you are going to take a step back and allow dad to deal with the situation. In the end dad needs to stand up and deal with his sons actions now.
It is a sad fact that these things happened to the child.........but allowing him to use this as a excuse will only cause him greater pain one day when he has wasted his life and has nothing.