Sounds like a classic case of hives.

It is an allergic reaction but you may never know to what. If it started at home mom will have to look for things like changes in laundry detergent, wearing clothes that haven't been washed yet, new foods such as raw foods not eaten before. Outside any number of bushes, plants, insect contact, something in the soil.... As I said you may never know what caused them.

He should be given benydryl allergy formula. That will help to control the histamines raging around in his body right now. Won't stop the hives but should slow them down. At the same time not an issue if he doesn't get it.

Encourage him to drink and pee lots as that may flush some of it out of his system but no guarantees.