Why would a child welt up?

It could be stress or he could be allergic to something around him.

It sounds like he is having an allergic reaction to something. Has he been exposed to any new detergents, or molds? Something that would have come into contact with his skin, perhaps?

maybe,you should just contact the doctor and ask them,what they think it could be.

It depends where on the body the welts are. If they are around the cuffs of pants, neckbands, sleeves, or underwear, it is probably a contact reaction to a detergent or fabric softener. If it is on the face or torso, it may be hives, which is a reaction to a food usually. Stress reactions have no discernible pattern in position on the body or when they appear. When I have stress, I get red marks on my wrists that itch and then I scratch and get red welts. Or maybe I don't realize I am already scratching before I notice the itch.

in some situations it can be caused by stress but also can be caused by food allergies or viruses in the body. maybe it would be safer to tell someone to take him to the doctor to get checked out just to be on the safe side.

yes