Welcome to healthcare Magic!
The skin blister
on the arm is mostly due to gram positive bacterial infection through the sweat glands, another possibility is insect bite usually also bear a bite mark that is on the center of the blisters and has a necrosed dark skin point.
Mostly the problem is self limited, just keep it dry and when the child has a bath, mop it with sun dried towels immediately to limit the time of being wet.
if more crops appear only then you need a medical attention.
Normally the lesions heal in 4-5 days, applying local antibiotics is unnecessary and delays healing.
I Wish your son a great health!