Just be as steady a presence in his life as you can, even at a distance. Remember those holidays and birthdays. Make phone calls (he's still kinda young to talk, I know). He's being raised by folks who're more suited to the job right now, and that's the right thing for him, but you can stay as in touch as much as possible so he knows you care now--and will know later. When he gets older, everyone can help him understand why it was important that he be raised by your dad and stepmom. I think that was a very mature thing you did to let them take care of him while you finish school and finish growing up! I hope they're good, loving, caring "parents" to him.