Men's shirts

If you want cheap and cheerful you could try the Jolliman catalogue their shirts are generously cut and look OK to start with but don't wear too well so would probably be suitable for a single wear if you didn't want to splash out.

Through illness my husband is that shape. He tends to find that M & S regular shirts fit him best. They seem to have a roomier cut through the body and sleeves. Hooe you enjoy the wedding. X

Our branch of Next have sample shirts on a rail for customers to try on so that they can check the fit before buying.

What about a roll neck rather than a stiff shirt with buttons.
Big wigs often wear T-shirts with a suit.
I think a nice roll neck would look good.
And much more comfortable and easy to get on.
All the best.

Charlestyrwhitt makes shirts with elastase so they stretch, maybe that would work?

Can you go to an independent gents outfitters or bespoke tailors and explain your quest?
Alternatively, traditional mens shirts were usually sold in collar sizes so if you went up a size or two would that give him more room?

I would look for 4 way stretch fabric.
I had a similar problem recently just trying to buy a long sleeved polo shirt for someone skinny but now with a bit of a belly through disability.

Could you find one of his good fitting shirts and insert a "triangular" panel each side to provide some room?

Shirts I discovered come in skinny, slim, regular etc.
Previously skinny he needed regular. It's a little bit baggy at the chest but maybe you could look at that if you don't want to alter things?