Both of my nephews, as well as my 4 year old son have had the rash issue at the beach. It's probably because their skin is softer, and more sensitive. It will improve as he gets older. This is what worked for my son just a few weeks ago when we were on vacation.....I put vasaline on the inside of his legs, or you can try aloe vera gel as well, before heading out to the beach, and it worked beautifully. (It seemed to be the salt water that they all had a problem with.) The gel acts as a barrier against the salt, (or the sand), and hopefully will keep your son rash-free!