Not only do you have the problem of how to get rid of the blisters you must determine how and why he has them. That has got to be a terribly painful thing as any time he eats (or probably even swallows) it must hurt. Your son does not need a "over the counter doctor" (layperson) such as myself. He needs to see a real doctor who can properly diagnosis his problem and cure it. I would highly suspect that he is drastically allergic to something which could (in the next encounter), lead to death if you do not identify the offending substance and help him avoid it. As a general rule of thumb, each subsequent reaction will worsen it's affect. The next time, it could well close off his esophagus. Take the child to the doctor.