How old is your son?
I think it's good to set an expectation that his reward is the snack at the end.

If there isn't a snack, take him out for ice cream.
Sorry -- I hate getting medals and I think they are partially responsible for the increase in prices. If you do a small, local 5K, chances are it's to raise funds for a small organization. I think it's great to run for the fun of it and it would never occur to me or my kids that a medal was necessary.
I do think if you go for BIG races, like others suggested, you'll be more likely to get a medal. Sometimes they mention that on their website.