There can be multiple causes for rash
usually have associated fever ,your son has only tiredness
2)it doesnot appear like chicken pox
because again in this condition you get high grade fever and crops of fluid filled rash.Hope he has received the chicken pox shot if he has then again the possibilty is negligible.
3)eczema or allegic rash coul be a possibility .does he itch?
which he suffered in Jan should not recur because of antibodies
in the body against it.
I think at present apply calamine
lotion .Wait and watch if the rash progresses and if he develops fever get in touch with his docter for evaluation.