It sounds like whiteflies. My son was about that age last summer and got bit horribly - you can't even really see them because they are smaller than fruit flies and white. I would call the ped to bring him in, because they may want to put him on an allergy med.
For awhile, I used Badger Balm all natural insect repellant whenever I took DS2 outside, but this year he doesn't seem affected by them. Is your son very pale? Mine's pasty white and the ped said that the darker you are, the more the melanin is able to kind of sheild you against reactions to the bites.
Good luck and please update!