I'm suffering from this horrible itching thing as I type this right now. My back was the affected area and when it first started to itch I tried lotion because it felt like tiny pieces of dry skin irritating my burn. That was a mistake as the lotion made it worse. To the point where I couldn't do anything. It dropped me to my knees and I was screaming in agony from this horrible itch. After the lotion i tried some "after burn" cream that i got from the store. Again it only helped to make the sensation worse. I had to run for the bath tub and I laid down in hot water. (as hot as you can stand it) it will hurt a lot at first because of the burn but the longer you soak in the water the less the dry skin makes you itch. Until you get out that is. I got some benedryl and that combined with soaking in a hot tub very often has brought the itching back to a somewhat tolerable level. These are the steps that I have been and will continue to use. Its not a 100% fix but with a sunburn that bad nothing will be except time.
1. Soak in warm/hot water for a long time
2. Use some benedryl but be careful because it will make you very drowsy
3. Never forget the sunblock again
I hope this helps and I'm so sorry that your son is feeling this right now. I wouldn't wish this on my worst enemy.... well maybe my very worst enemy lol. Good luck.