So as not to drip-feed: we have started the assessment process for ASD, which I think he may have, and one of the factors is that he is 'socially immature.'
When it's hot, he likes to take his clothes off. I think that when he's in his own home, with no visitors, there's no reason why he shouldn't. I certainly don't want him to think that there's anything shameful about his body - I have explained to him that quite a few social taboos are a bit silly when you look at them logicaly, but the point of following them is to not make other people feel uncomfortable. Therefore you can be naked at home/in private, but not while running round the park.