I agree with the previous posters that it is probably easiest to just let it go. But I know in your position it would really eat at me as well because I would just want to know why, lol. I wouldn't ask them about it as such, but perhaps you could just mention to them that your son mentioned that it's their son's birthday soon and that he would like to celebrate it with him, so you would like to invite him over for a playdate. They might open up about the party and you could find that they just totally forgot your son or even thought that he WAS invited (considering the son made the invitations himself...).