Hi,
What you describe seems like sweat dermatitis or heat rash.
You should try to keep the area dry and clean. Try some powder to absorb the sweat.
If the symptoms are still persistent, consult your dermatologist and get a proper clinical evaluation done.
Let us know if you have any doubts and keep us posted on how you are doing.
Good luck.
allergic to the sun perhaps? I've heard of exercise induced hives...I hear Zyrtec is great at controlling hives associated rash - maybe he could try that and see if it helps?