Thanks for your query,based on the facts that you have posted it appears that your boy friend has blisters on his glans penis
This is due to infection of the glans ( Ballanitis) This is definitely not due to sweating .
Microscopic examination of a fluid from blister ( Swab Test)will help to find out the cause of the blister.
He needs to take broad spectrum antibiotics like Cefotaxime
along with anti inflammatory medication like Diclofenac
twice daily and topical local anaesthetic
ointment like Neosporin
Ensure that he washes his genitals with warm water twice daily.
Avoid sexual activities till it gets cured completely.