I have not read everyone else's posts so I am sure there's some crossover.
DS has Sensory Processing Disorder (and, as it turns out, ADHD, but as a toddler it was the SPD that was the key issue).
First, what is quite scary is how even at such a young age, they learn to mask. You say he'll do it for other people.... but look closely at those situations and ask yourself if he's really happy or does he find excuses to come home early or to do something different? DS was, frankly, a bit scared of my sister so would dress or whatever for her... but the first moment he got, he was back to taking things off for finding sneaky ways round it.
Second, remember that with SPD it can be the feeling of the clothes, yes, but also, the feelings involved in putting clothes on/off. So, for example, in DS' case, anything that went over his head he would resist massively. And we only realised quite how bad it was for him after we had another baby and saw the difference. So once a t0shirt is on, he'd want to keep it on for DAYS if he could, simply to avoid taking it off and putting on a new one.
One way round this was pyjamas that could be worn as t-shirts. We had a large collection from places like Character.com - star wars, spiderman, peppa pig etc - that were long sleeved and he'd wear them at night, then into the next day and take off at bath time.
Third, layers and restrictions can be especially unpleasant for them. As a result, yes, DH adn I were those parents with the child wearing a pair of trackies and a long sleeve t-shirt coming in and out of the soft play with no coat on for the trip back to the car etc. You really do have to sort of accept that other people can judge you all they like... you're doing what's best for you and your child.
Four - if you find things he likes and feels comfortable in, let him wear them. Also, if there are particular areas of resistance, see if you can identify what they are. Our life was completely transformed when we switched to seamless bamboo socks for example. getting his shoes on and out the door was SUCH a nightmare. Until the switch. Then suddenly life got easier.