My husband has been able to rent complete outfits in his size at Men's Wearhouse, no trouble. This despite the fact that he has a hard time finding men's shoes in his size at most places.

One thing you should do is make sure you leave several days between when you pick up the tux and the night of the event. They have never done the right alterations for my husband first time. In one case, we received everything unchanged, because the tailor thought the measured proportions were a mistake. If you have extra days before the event, you can send things back for further alteration and still get them back in time.

I might be over-complicating this and worrying too much (haha me? 😅🙃 never!)

Haha you worry cause you care!

but heat might be an issue?

It’s totally normal, after pictures and the first couple dances, to lose the jacket and the cummerbund (I think that’s what it’s called?) and the unbutton the top couple buttons.

One thing that's nice about a suit or a rented tux is that they should be able to mix and match sizes. It might be possible to rent something from like a Men's Warehouse without going in person if you can take measurements. Suit jackets in particular come in long, regular, and short lengths in addition to the numbered sizing, which might help find something that fits well. I'm a short, stocky build so I've always struggled to find well-fitting formal wear as well. Suit separates are also good in this regard, since the size can be mixed and matched.

As for the extra pieces, I think it would be best to go with no vest, cummerbund or nothing instead, and bow tie over neck tie. Both a vest and a neck tie are killer on the heat equation. Bow ties don't have to be nearly as tight as a neck tie so it's way better on the heat and comfort. One you tie yourself can be an especially nice touch as people begin adjusting to the warmth.

A tip for the pants if you have to get too large a waist to get over his hips: suspenders. A pair of clip suspenders will help keep the pants up if you have to get a size that's too large in the waste.

I think it's important to recognize that even at this age, both cis and trans men have all kinds of different body shapes, and the average cis male dresser doesn't think about ill fitting clothing at all. You are way ahead of the curve thinking about it now, and the resources available should be sufficient.

Just my personal experience renting suits for a friends wedding recently, half the dudes at the suit rental place in my conservative little hell were very flamboyantly gay and the other half were 100% style nerds. I think its also more and more common for AFAB people to wear suits, hell the maid of honor did.

I would take some time to scope out your local places in person and maybe get a quote or two before jumping out to a main city hours away, you should know right away if like when I was a kid getting my first suit for prom and it was an old dude with awful breath at a mens warehouse.

Honestly buy one.

You can probably even cop one cheap at a second hand shop, they get worn once a year at most so are usually in pretty good shape. Even new it'll pay for itself in two uses. Used it might even be cheaper than a rental and you can branch out into colors besides black. Navy always looks great, and of course a white jacket is peak performance

Don't overthink it, find one that sort of fits then if you feel generous have it tailored. Black dress shoes are fine (shine them up!) and for the love of God and all that is holy get a proper bow tie and google how to tie it. Clip ons look ridiculous and an untied bow tie is a party accessory must. Cumberbunds or whatever the hell they're called are hella dated, suspenders all the way. Top it off with a pocket square. He will look dapper!

If heat is an issue, then you should really consider the material it’s made out of. Linen or cotton tend to be the coolest but if you don’t like the aesthetics of them a lightweight merino wool one also does a great job at regulating temperature. Just stay with natural fibres.

And with sizes, well... trans people tend to have problems with standardized clothing because our bodies are just not standard. I have 3 different sizes across my shoulders, waist and hips. Thus I have to make compromises with clothes that cover more than one of these areas. And standardized suits, which cover the whole body, are already a problem for cis men. So I would guess that your son is likely to run into problems as well.

But the good news is: Nobody really gives a shit if a teenage boy has an ill-fitting suit. Mine was too big for me and I was fine. On the other hand, if he really wants a well-fitted suit then there are definitely options. One of my relatives is a cis man with REALLY big hips and he manages just fine. It just takes a lot longer to find the right stuff and you might need to buy one and have it altered. Or you get a custom one, but that will be pricy.

One thing with tuxedos is the the pants are traditionally meant to have a full break—basically, the pants drape over the shoes to a pretty significant degree compared to the usual these days

Not entirely what you asked, but does he have shoes picked out? If it's to go with a tux, I'd recommend black loafers as they'll probably be comfier for dancing and such than the traditional oxford dress shoe. Opera pumps are also an option, but might be too fem depending on his style preferences