Well said, gcvmom. It could simply be sensory, and if you approach it from that point of view you might also sort out any other issues he is otherwise reluctant to share.
Serious suggestion, though - why not do what difficult child 3's friend's mother did, and get your son some pyjamas (or something) exactly how HE wants them? Although from the sound of it, you're already doing the underwear for him (his, not yours).
I remember difficult child 1 wouldn't wear some of the school uniform shirts I bought him because the seams were too scratchy - they'd been overlocked with a synthetic thread instead of cotton. Also, I had to remove the labels because he said he was too distracted by the scratching of the label on the back of his neck.
And one more thing I remember, could be an issue with him borrowing your underwear - the smell is important for some kids. difficult child 1 would get upset with me when I raided his bedroom looking for washing, and grabbed the tracksuit he was wearing as pyjamas. He complained that I had washed out the smell and it would take him ages of wearing to get them how he wanted them again! Not a BO smell, either - just HIS smell. I have a very sensitive nose so I understand. But his must be even more sensitive.
Another thought - you know how some kids with Pervasive Developmental Disorder (PDD) like to feel pressure? Like easy child 2/difficult child 2 and her corsets. We had a weighted vest made for difficult child 3, it did help calm him. A simple pattern - like a photographer's vest, with lots of long, thin pockets (two in the front on each side, four across the back) and in each pocket you put a long, narrow sand bag weighing 200 g. The vest and pockets were double-stitched with french seams, for those wanting to try it. The vest fastened down the front with velcro and was available in a number of colours. He was in mainstream at the time so I requested a colour that would fit in with school uniform. But it could be worn under a shirt.
difficult child 1 experimented with wearing weights too, he said it felt especially good when he was lying down and had the weights on his chest. The thing is, THEY need to control the feeling of pressure. That way they feel safe and they relax more.
Marg