1st of all, it IS VERY VERY hard for a child around that age hearing how all these kids their around (friends or family) having their dads do a bunch of stuff with them when they don't have their own dad...i know, my father died the mother's day after my 1st birthday (less than 3 months after) and i also seemed to always have an attitude when it came to my mother having guys around that weren't just friends (i KNEW when a guy had more intentions then just being friends...even though NOTHING ever happened while i was awake or around between them). my mother pretty much kept them away from me and my brother (who's father was still in the picture, but just through visitations kind of deal). out of all the guys that my mother brought home (which weren't all too many thinking back now, especially over 20 something yrs!) there were only 2 guys that i liked, 1 didn't work out because he decided to go back to his ex and get remarried (which my mother just stepped aside not wanting to be in the middle of that) and the other she is with now...though it took a couple yrs for him to get my approval, and ALL but those 2 guys were horrible men. whether they had some addiction (whether drug, alcohol, or gambling) or they beat their kids (my step-father) or they were just all around assholes...and my mother NEVER saw any of it until the END of the relationship...they hid that part, but kids KNOW sometimes!
as for the other kids with fathers, it's going to be hard, for me i would cry when i'd come home because i knew i was missing something special and important not having my own father, and refused to allow anyone to substitute him...so that's just something he's going to have to work through as he gets older, there is no special way to make it right. being that his father walked out, and didn't die, you can't just tell him how wonderful a man he was, and tell him everything you can about him, then try to have other men (grandfathers, uncles, etc.)in his life to try and substitute (though that was my situation, and it didn't help anyway..but i'm different) and you can't just tell him that he's better off without him because he's a jackass who just LEFT (didn't make anything better for anyone i know who's mother/father just up and left them at a young age) it really is something that he just has to work through. honestly, therapy may help...ask him how he feels about it. let him know that you realize that it's hard for him, and it's HIS choice if he goes to therapy, but it's helped some children that i know now who have had 1 of their parents up and leave at a young age and are now watching all their friends around them with their parents, having what they want...just their mom or dad. sometimes therapy doesn't work, you'll know if you take him..but it may take a couple times, maybe a couple months before...just don't force him, because it can make things worse, but if he wants to try it for himself, it could do wonders! i wish you and your son the best in getting through this.