It would be very difficult to say anything without having a look at the lesions.
Is he on any medications currently? This could be due to an allergic reaction to some unknown substance or secondary to insect bite or sweat dermatitis etc. But nothing can be said with surety without a clinical examination.
You should wash the areas and keep them clean, apply calamine lotion at the site for his symptoms.
It would be best to consult a skin specialist or pediatrician if the rash does not resolve in a few days. You should get the relevant investigations as advised by the doctor including blood work.
Let us know about what the doctor advises and if you have any other doubts.
Post us about how he is doing.
can break cause sress its mosty alll on my back legs and arm and i was asleep woke ut to a rash but it happend to me one day last week just sittin o my chais and there gose the break out and i didnt eat anything what could it be