It is very difficult for mixed families to come together ("his", "hers", "theirs"). There is no simple solution, however I'm sure there are a ton of books out there on the subject. But I do believe the true answer/key is the unified front between you and the boy'* mother. You and she needs to be on the same level... the same understanding of how things need to be, even if you are not living together (and especially when you do). Actually, it has to start NOW, before you move in together.
While you are not his father, the boy does need to respect you, her, your home, etc. This respect will only come if the the mother promotes it. Therefore, it all starts with your relationship with her.
I urge you to research this online...there are tips/tricks...do'* and don'ts... it'* not easy, but it is attainable, from what I've seen of others. Communication and mutual support from both the mother and you will dictate your relationship with her son.