Hi! My daughter used to disrobe at night before falling asleep in the winter months as well. As long as it's not an aversion to the fabric or something along those lines, I suggest putting him a footless onesie pajama on backwards. This solved my daughter's attempts at disrobing and removing her diaper. She wasn't able to pull down the zipper since it was down her back. After days/maybe weeks of doing this, try regular clothes, the hope is that he forgot and gets past the behavior.

Ah...the Early years...
First, you may have to change the brand of fabric softener or soap that you use.
Make sure that the clothes are Soft to the skin.
Lavender, the smell of that helps calm.

.My son used to do that a lot too. It caused some problems at school. I think he liked the attention and big reaction he got from people. At home, I tried not to laugh or give a big reaction, stay nuetral and calm and just keep the demand to wear clothes. If he refuses to put them on, I wait 3-5 minutes and try again unless I know he has diaper rash that was uncomfortable. He has grown out of this but it still does happen on occasion.