Yes, some people are very sensitive about soap. Boys, girls, intact and circumcised. Why don't you suggest to him that when he washes his penis he makes a habit of doing it first thing when he gets in the bath before he gets any sampoo or soap in the water. And try to make the soapy part of the bath come at the very end so then he is not sitting in soap the whole time. You'll probably want to avoid bubble bath too.
I don't think the stinging is a sign of anything wrong with him, just his normal sensitivity.