Can you help my son with a sensory issue?

Hoping there are others here who have experienced this and might have some clothing or product recommendations. This is a private area issue, but we've already tried a few things and I'm not sure what else to try. My son is 6 yr old and was diagnosed with ASD last year. One of his sensory issues is he doesn't like it when his private parts are sticking together. "Sticking" is how he describes it. The problem is this leads to him touching his crotch to make adjustments A LOT. I have talked to him about doing this before leaving the bathroom and when he gets out of the car kind of using the door for privacy. Now I want him to be comfortable, but I also don't want him to get made fun of or for adults to constantly be worried about him. His own grandmother refused to believe me that he doesn't have a rash, that's how often this is happening.

Anyways, we've tried "quick dry" boxers, bamboo boxers, a powder called "Happy Nuts", and some quick dry cargo pants but they were cheap so not sure how much they helped. The obvious choice to me is to wear loose fitting athletic shorts, but my son won't wear shorts. I'm willing to try a nicer brand of pants if someone recommends them, but since he is picky about clothes I didn't want to start with that.

Has anyone else had this problem? Are there certain brands of quick dry pants or underwear you've found helpful? Or other products?

Thanks in advance for the help!