Well, sorry for a long delay on anyone responding to this here, heh.

Most of my sensory discomfort that occurs in that region is based on underwear. Essentially, I have a hard time with underwear that has seams, tags, and elastic bands. Standard briefs generally aren't good because of the top band and sometimes the leg cuffs. Standard boxers are out of the question, although I used to wear them to much discomfort for years, because of the crease that they have running right down the middle. It irritates me to to no end. I always have to make sure my 'bits' are aligned crooked to avoid the seam.

This would lead to a lot of me trying to secretly fidget with my underwear all day. One of my middle school teachers actually pulled me aside, told me people had reported me doing that, figured I was masturbating or something, and told me to quit that and 'control my urges'. I laugh thinking back on that, lol. I was just trying to be comfortable. I told him that, but I don't recall if he bought it. After all, I didn't know about SPD back then, and wasn't able to use that in my reasoning.

The only kind of underwear that has worked for me has been boxer-briefs. Not just any kind though; they have be the kind that doesn't put the seam down the middle, doesn't have a tag, is made of a sensory-friendly material, and (preferably for sure) doesn't have an elastic waistband. Sure, the kind I use has a couple seams, but they run down the front and back in areas that cradle that region, rather than running down the middle right where everything is, hah. Ever since I switched to using those, I've practically never had to dig my hands into my pockets to 'stealthily' deal with discomfort issues. I simply am not so uncomfortable in them.

Now, you say he's saying this about the 'inside'? It's possible the underwear isn't the issue, and that he does have something going on internally that is causing this. It could still be sensory-related, especially if it is a proprioceptive or interoceptive issue (internal sensations related to muscles and body functions, respectively). For things like that, my best guess for treatment would likely come along the lines of deep pressure, and possibly reflex integration and listening therapy, as unrelated as they may seem. Ultimately, I'd recommend talking to your son's OT (assuming he has one) to see if they have any valuable input or ideas on this subject.

Of course, this could always be a non-sensory issue, so it is important to keep up with trying to find alternative medical reasons for this, especially if the sensory-treatment approach isn't helping. Hope this helps you in some way.