Does is EHCP specify that he needs help with dressing/undressing? Does it fund a full time 1:1 for him?
I say that because the response can be very different depending on the answers to those. If his EHCP specifies help with dressing, you are right to be a little annoyed (although I think furious is too much, in all honesty). That being said, I have Asperger’s and am very sensitive to clothes being the wrong way/inside out/labels etc. - I may not have outright said it as a child, but I’d be letting people know through wriggling, scratching and not paying attention that something was wrong, so if he isn’t able to verbalise his discomfort, are there non-verbal cues someone should be picking up on? Or was he genuinely not bothered by back-to-front/inside out and, as gently as possible, was it more of an issue for you?
Again, if he has a full time funded EHCP, then a member of staff should be with him all the time. If it’s only partially funded, he may not be allocated time for that gym period. If the school is ‘topping up’, for want of a better way of putting it, his hours to be mostly full time, I’m afraid you can’t insist he has someone all the time. Schools have to fund the first 15 hours of an EHCP from their own budgets, which coupled wih issues around funding from the high needs block, means staff have to be spread more thinly. Unless his EHCP specifies PE and/or dressing and undressing, then it might be difficult to ask that he always help for this. Unless he has a fully-funded EHCP, in which case ignore all of that. 
Have you asked him about his clothes being inside out/backwards? Would he tell you if it did bother him? Regardless of what other posters have said, he is highly unlikely to have been being laughed at or mocked by his classmates at 7 - looking like you’ve been pulled through a hedge backwards is still pretty common at that age.
Perhaps have a word with the teacher/TA to focus on one thing in the last couple of weeks - trousers on the correct way, for example - and to make that a focus?