My life has changed and I'm not happy. My 18 year old stepson has moved in with me and my husband.?

His mum could no longer cope with him. I don't have any kids and never wanted any and this is something both my husband and i wanted but we were happy with the old arrangement. We had regular weekend visits and holidays through out his childhood and we have a pretty good relationship - that is until now. My husband seems to be trying to make up for lost time so now he is invited out every time we go out with our friends. These are friends with no children or step children. I have begun to feel resentful since I have to share our Friday night with friends after work (something we have done for 13 years) with an 18 year old. I have explained to my husband that we need this time back for ourselves and he understands but hasn't actually done anything about it as he says it's hard for him to tell his son. Please help - I no longer get any private time and it's driving me mad! Am I being unreasonable?