Sorry the links didn't work but yes, it was a leather jacket.
I really feel for him and it was a similar issue with DS2. He just looked like.a sack of potatoes in a suit.
Something reasonably fitted and not too sack like would be a better option than something baggy honestly. It's difficult with shops at the moment but going in and trying a range of things would have been a good option.
It obviously depends on where his weight is distributed as to what the best look would be, but slim jeans would be better than loose and a decent length shirt would be good. The one I linked to was a grandad collar which might fit better with his style?
He could completely go without a jacket but I still think some sort of bomber type jacket, leather or otherwise, would be a decent option to try.
I know chinos and shirt are the go to option but neither of my DSs would wear that and your DS sounds most in line with my DS2, long hair, band t shirts etc and he wasn't an xxl but has been XL and very self conscious about it. So I've tried to recommend things that I think he would wear as your DS might like similar.
I think feeling good in what he wears is more important than fitting in with a stereotypically wedding type outfit.
Pick somewhere with a good return policy and order in a range of options for him to try on at home. MandMdirect might be another source of things?
I find Next decent as I have an account to get free delivery and returns so I can order in multiples. They do some particularly flexible jeans which DS really likes. He's a 36 waist now as he lost a bit of weight. he still gets the skinny fit though and they look good on. He tends to go for a dark grey or black.
I really hope you find a nice option for him, we all deserve to feel good about ourselves.