Bashing, yes I do know what it's like, but I have a DH, who is DS's dad, so no issues there, although there have been times when they have really clashed and DH has threatened to throw him out. However, things always calmed down and were fine in the end. I have had another child when DS was 17, so every time he's come home, he's had to sleep on the sofa bed in the playroom. It wasn't ideal, but we made the best of it.
I know that when I was growing up, I left home to go to Uni, and could always come home to my parent's house. After Uni I traveled, and at age 24 came home to my parents, broke and unemployed (I soon got a job and left home). There was no question of whether I was welcome, it was my home and I knew I could always go there, no matter what. My older brother lived there until he was 27, but he did leave, so it wasn't forever!
My 20 year still lives with us, is fully employed and has had stints of living out of home, but honestly, I love having him live with us. He pays board, and does a lot to help around the house. It is actually really nice having another adult in the home, and his younger siblings love having him here too. When he is ready he'll move into his own place, he's already talking about it.
It sounds like your DP has an issue with your DS and it is important that DP behave like an adult and talk it over with you. I don't know the details of your relationship, so I don't know if it is a deal-breaker, but surely if your DP loves you he would realise that it is worth discussing the issue. If he can't do that, then, yes, give him his marching orders.
I know the drinking is a separate issue, but I'm not sure I'd be trying all that hard to stay with your DP. That's a lot of wasted money, and you know it will really affect his health in later years, and you'll be the one taking care of him...