You could always check with the teacher, I suppose and ask her/him if there is anything they would recommend or anything the kids have had a lot of lately. I never think to do that in time.
Cereal with milk either in bowls or cups is fun, you can choose a healthy cereal.
Mini muffins are healthy (sometimes i wonder though) and they look like cupcakes you could dust them with powedered sugar.
Kids love banana bread.
Popcorn is a pretty good treat too. slight choking hazard for wee little ones. so depends on age.
Furit Kabob's on cute fancy toothpicks, might be able to get leftover new years ones cheap right now.
And i'm sure they don't get jello cups or pudding cups often, you can get those sugar free, but you might want to check with the teachers depending on age just because of the mess. Don't forget the spoons.
How cute would Jelly sandwiches (sans the pbutter) cut out with cookie cutters in the shape of a dinosaur or what ever he is into right now!! with some lowfat choc milk mmmmmmm.
I saw something i'm going to adapt a little and use for my daughters birthday snack. I have no idea where i saw it or what exactly it was filled with but I'm going to use Sugar Cones, that you would put icecream in and put either popcorn or some kind of trail mix. I'd like it to kind of mound up over the top but i'm not sure how to get it to do that. I'm still thinking about it.
familyfun.com had some cute snacks sometimes
Happy birthday to your son. I'd love to hear what you decide to do.