Teresa:
Any rash that comes and goes that fast is more likely to be prickly
heatBabies and heat rashes
Heat emergencies
Heat rash
Infant heat rash
Tenosynovitis than allergy. In any case, it isn't something to be concerned about, unless it bothers him more than it it probably does. Just use a broad-specturm (UVA/UVB) sunblock (a hat is even better!) and pay no attention to it, is my advice.
Best.
Dr. Rockoff