My little girl is just the same way, and there is no quick fix. She hates jeans and socks with seams, underwear that are too loose, and the list goes on. We've had her evaluated ,and she doesn't have sensory issues with anything else, just the way some fabrics feel. What I've learned is that the issue with her and these fabrics is pretty deep seated, meaning that if I force her to wear them, she is REALLY uncomfortable, struggles to focus on anything but how badly the fabric feels. She never throws tantrums or anything, but you can see it in her face, desperation to get the clothes off. She's told me that she feels like the items are suffocating her. At nine years old, she's more willing to try clothes that have bothered her in the past, but still avoids jeans and seamed socks whenever possible
Now, I let her pick out many of her own clothes-with my gentle guidance of course. My daughter wear leggings alot, and has some nice athletic and "yoga" pants. When it's cold, she'll wear tights or flannel pants underneath. Children's Place makes jeans with soft fabric lining the inside, you can probably find those online. We buy our socks at Target or Old Navy-no seams.
From what I've experienced, this isn't rebellion or defiance. These can be real issues for some kids. I wish my daughter would wear jeans and not be so picky with socks, but I understand her struggle, and I'd rather have her comfortable and not full of anxiety (and she would be) over her pants. My only rule with her is that she still has to look nice and it must be weather appropriate. If she chooses to wear athletic pants and leggins in the winter, she must wear tights or long socks underneath. This is a fair compromise for us.
It's a tough thing to deal with, but if your son is like my daughter, it's truely a difficult thing for him, he's probably having a real physical reaction/feeling from these things. Try to be sensitive, there are so many alternitive options for clothes out there that aren't expensive. I would still have him assessed by a specialist just to rule out any sensory issues, but rest assured, for some kids, its just as simple as a clothing issue.