It is very hard being a step parent. You try and build a relationship with someone who does not really want a relationship with you and is influenced by outside forces which are totally outside your control.
My relationship with my DSS was great at first, till he realised that my relationship with his father was serious. Then he felt he could not like me as his mother did not, felt he had to be loyal to his mother.
A few years ago, I arranged for us all to go skiing for a week. We had a great time, he loved it and was a good little skier. Within 5 mins of us being home, I heard him on the phone to his mother, complaining about the holiday, the apartment, having me and my daughter there etc. I then realised that no matter what I did I was always going to be the bad guy.
What I am trying to say is a lot of the factors that will influence your relationship with him will come from his mother and her relationship with his father and her attitude to you. If he is only with you half the time you will have to put up with the bad stuff and concentrate on the relationship with your DP. Any aspect of his personality that bothers you now will not go away and will possibly become worse when he has to deal with you marrying his father etc. His father will also feel guilty about his son, so will try and make up for that and be easier on his son than usual, and defensive.
What I try and remember is that it is not really personal, he would be like that with whoever his father is in a relationship with.
What you need to remember is that you are unlikely to be in a relationship with a man who will have kids you will love and get on with from the start, nor a man who does not have children from a previous relationship. So if you have found someone who you love, then put up with the problems of step prenting and do your best.