Not that unusual. Both my son and daughter had similar problems when in their teens. Son one armpit, daughter both. Several of their friends also had the same kind of problem as well.

Your son needs to see a dermatologist about the problem. Your son's attempts to help the problem will accomplish nothing.
He needs a prescription type of deoderant/anti-persperant to help the problem, along with a good antiperspirant deoderant.

Let the dermatologist determine the problem, prescribe the treatment and son's problem will be gone in a few years.

Both my kids use mennen antipersirent/deoderant. They have a one that goes on clear and stays clear. It is a really strong one to use and most oncologists tell their patients that are undergoing chemo and radiation to use this one as it works the best. They can have the same kind of problem

My son's problem began when he was 14 and was gone by 19 with treatment. My daughter's began at 16 and was gone at 20.

Good luck