What laundry detergent do use? I have son that suffers from chronic hives and I have all free and and scent and dye free fabric softner.
As for what he is allergic too, many allergies progress over time so he may be showing symptoms things he has been exposed to for a long time and in rare cases like my son it may not have any real answers. He has a mild form of sensory disorder and his hives appear to be stress related, now the tricky thing, what causes the stress. Well for him everything, noise, sight sound, touch, heat cold. I use the softest sheets I can find and always had issue with clothes. Especially around collars waist band. Always try to find the softness, no tags etc.
He is older now and has grown out of much of it, it didi get worse before it got better and we had to hav e him on low dose allergy meds everyday for several years.
I would suggest getting a referal to an allergist, you may get answer you may not, but you should be at least be able to find some kind of plan to help him through. When they are itchy liek that all the time the end up cranky which can lead to behavior issues, and bad habit s cna be formed during that time.
Good luck poor guy I have so much sympathy for him.