Poor guy. I agree, if it's not near the sunburn I don't see how it could be related. I get this way myself sometimes, and it's usually because I switched detergents, bath soap or body wash, anything that touches my skin. I'll look and there's also no rash, but it itches like crazy! It's called
"contact dermatitis." What your doctor recommended is good treatment, though, but I use an anti-itch spray, it irritates my skin less than the cream. Do be careful with the Benadryl, that stuff scares me and we don't use it in oral form at all, but in the spray and gel for the kids kids. Oatmeal baths may help relieve the itching, too.
Think back to figure out if you switched detergents, soaps, fabric softeners, his bubble bath, or put a new article of clothing on him before washing it (the sizing in them irritates many people.) If you figure something out go back to the original and hopefully he'll stop itching soon.