Your son has some major growing up to do. Right now, his girlfriend’s mom looks like a whole lot more fun than you do. Of course, that’s because she is as immature as he is. Unfortunately, she gives him justification for writing you off.
You are under no obligation to support him now that he is out of high school. You and your husband need to come to agreement about what behavior you will and will not accept. Be absolutely clear about it. Ultimatums are only powerful if you really, really mean them.
Then, if you can bring yourself to do it, inform your son that you will only continue to support him if he is in school and getting decent grades or if he is working and paying you a reasonable amount for room and board. Remind him that you deserve to retire at some point and that you’ve done your job of parenting him. If you decide to go this route, it will be hard to stay firm. You do love him. You don’t want to alienate him. But sometimes tough love is the only thing that shoves someone out of taking advantage of a situation.
Do understand that he may have a tantrum and move out to live with the girlfriend. That’s okay. A little reality is good for him. My guess is that the mom will eventually tire of supporting him financially and tripping over him in her house.
Do let your son know that you love him and want what’s best for him. Remind him that if he is willing to commit to being responsible, you are willing to offer him a home. If he chooses to move out, let him know that you still love him (it’s only the truth) and that you certainly hope he’ll drop by once an awhile.
Transitions are tough. You remember that from birthing him. Some kids seem to require another big push to get out and to get on with their lives.
I wish you well.