I'm trying to think back to when my dss was that age, he is now 9, from what I can remember he didn't interact with me much, but then even now he's not much of a talker.
I just took every day as it came, some days he would ackowledge my presence other days he didn't, if he wanted to be with his dad all the time then I just stayed out the way, it prob helped that dsd was there and she loves me so we just done our own thing. I never forced him to accept me or to even be nice to me, I was just there, I sometimes bought him little gifts (obv dsd too) and we took them to fun places which didn't cost the earth.
I hope I don't come across as condescending in anything I write, it's really not intended that way I'm just trying to help you as I know how hard it can be. Being a step-parent is a very hard thing to do, you might feel as though you are constantly trying to prove yourself, I would strongly advice that you be yourself around your dss, try not to get upset by anything they may say to try to hurt you, they are just children and are hurting in their own way and when your dss is at your home make it fun time, bake cakes and cookies, rent dvd's and get a huge tub of popcorn, on hot summers day get the paddling pool out and have a good soaking.
Just try and be his friend, if he wants you to play thats great, if he doesn't thats fine too, or even start doing something you know he will love and ask him if he wants to join you or help.
I hope my rambling is of some sense to you xx