Your son could change his coat to the exact same one that the ringleader has, and he would still be bullied, they would just find a different "reason".
It might be different with seven year olds — and it really depends on what's actually going on in the original interaction — but I was maybe a couple of years older than that when I discovered that immediately swapping the targeted item for something seemingly more acceptable was not necessarily very productive. I'd have shown them that they could make me do whatever they wanted, and they were gleeful about seeing that demonstration of their power. I'd then be mocked for complying, for copying other people, for thinking I could make people like me using my possessions, and so on, and the new item criticised. Then maybe they'd target something else, to see if they could get me to change that and then laugh about my compliance.
Even if, unprompted, I happened to turn up to school with something which was too similar to what the "popular" kids had, I'd get the piss taken out of me — why was someone like me pretending they could be cool, you look stupid with that bag, etc. etc. So, as a teenager, for example, I might fly under the radar with a fabric shoulder satchel, but a rucksack (especially if worn on both shoulders) might attract pisstaking for being sad or a geek, and a fashionable Jane Norman shoulder bag would've got me mocked for trying to be someone I wasn't. To some extent, most people did fit in with the expectations of their subgroup, but some kids could've got away with having something different, while others couldn't.
I guess there are times where it really is just about the coat, other kids find it funny, and once the coat is gone the whole thing's forgotten. Then sometimes this kind of criticism or mocking of someone's possessions is really part of a no-win scenario where the possessions are just the means being used. My experience isn't universal, I know that. But I agree that if it's a scenario like I experienced, the specific item can be a red herring.