When my dd was younger, she had a reaction to most sunscreens out there. We got to a point where I'd squeeze a tiny amount at the store and spread it on her skin somewhere, because it was just way too expensive to trial them all. Even with that tiny amount (yes, I felt horrible for doing it, as someone else bought that bottle for full price.. it really was less than a pea sized bit), she would react.
Finally, I took her to the doctor at a clinic, and he said he suffered a similar problem, and had found Ombrelle to be the only one that he could wear. I bought a bottle (very expensive) and forutnately, it worked. I bought the regular adult variety over the child version because there were fewer ingredients in the adult version. She used it successfully for years. Then one day we ran out. I skeptically tried a tiny bit of whatever brand my parents had on hand (a really cheap brand from walmart), and she didn't react. Both she and I were expecting red, itchy rash, but it didn't happen.
So, you might find a pharmacist who will let you try a bit of Ombrelle on your son before buying (it's expensive). But, my dd outgrew her reaction, and she's 11 now, so it's possible your son's reaction is here to stay, or more severe to begin with.
Good luck finding a solution that works for you.