I noticed that my son had red spots on his legs that itched him really bad, I saw them when he came inside from playing outside. What could it be?

Insect bites. Probably ants or chiggers. Use benadryl, (diphenhydramine) at a dose appropriate for his age/weight, and clean the bites with alcohol, or have him swim in a chlorinated pool for about an hour, and it will clear it up. If not better, needs to be seen.