DP has a stepson (aged 16) he's been in his life since he was very young and he doesn't have a relationship with his bio dad. He carried on seeing him after he split with his mum and was still supporting her financially. She met someone new and got married but she sadly passed away 3.5 years ago and he stayed with the husband. DP tried to support him but he stopped replying to him as often and he didn't really come here. He still bought him birthday and Christmas presents but he wasn't giving money to the husband as they didn't have much contact. When he turned 16 DP messaged him saying happy birthday and stepchild asked DP if they could do something. DP agreed and took him out and he told DP that the husband didn't get him anything for his birthday and hadn't mentioned his birthday so he'd probably forgotten. We tried to make his day as good as possible then we didn't hear from him for a while.
He's started sleeping over regularly the past few weeks and mentioned that the husband has a new partner that has moved in but that was it. He came here yesterday and was upset, he wouldn't say why and we didn't want to question him whilst he was upset so we left him be. Before college he told us that he was kicked out and wants to live here and before we could ask why he left to go to college. I'm unsure and think we should know why he was kicked out before we agree, I think we would struggle to afford him to live here fulltime but we would make it work by cutting back on a few things. DP thinks we should let him without needing to know as it's his business not ours. AIBU?