You don't need to be his father, he will resent you. Be a role model.
I was in a somewhat similar situation, in that I married a woman with a 10 year old boy when I was 28. I could not replace his father, and to try would have driven a wedge between us. I never hit him, and whilst I tried to guide him, I left the ultimate parenting to his mother. We have ended up as good friends as a result.
Teenage boys usually become difficult, lazy and rebellious. I had to realise not to take it personally with my stepson, and you will need to do the same. When he is difficult with you, it is not because you are his brother, or a fill in father, it is simply a natural part of growing up.
When he does stupid things, don't be too hard on him. That is how people learn.