Thank you for your query.
I can understand your concern. However, it is difficult to tell what the problem is without having a look at it. Since blister
formation has occured and it is getting worse I suggest you show your son to a doctor who will guide you furhter in this regard. In the mean time just apply some bland ointment/oil till you see a doctor.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.