My brother is 7 years younger than me and it took a long time for us to get on because we were always at such different stages. For example, he was in nursery school/infant school when I was going through puberty and didn't understand about my need for privacy and I was quite intolerant of him and to my shame, was quite nasty to him sometimes.
In many ways my mum has ended up with two 'only' children. From my experience, I would prepare your DS for the fact that babies are quite boring for a long time. I had wanted a brother or sister for a long time and when he did arrive, after the initial excitement, I remember feeling very disappointed by how little he could do. I also went through periods of being jealous at the loss of attention but that could just be me being a bit of a drama queen!
However, we do get on very well now (28 and 21) and during the time I lived at home after uni and he was going through illness, we were really close and a great support for each other. It just took a long time to get there.