how long as he had it? Does it seem related to his belt? Is it itchy/painful, etc? does it look like hives?
Could be contact dermatitis.
..but a picture would really help. If it is contact dermatitis, you can try Benadryl (diphenhydramine) cream and hydrocortisone topical cream. these are both over the counter.
Would you like to video or text chat with me?