Light gray sneakers (think fashion, not athletic) also would be fine. Or the shirt could be light gray with a pattern and the sneakers white. The issue probably is that yellow scares men and can clash with other colors.
This feels like a brainstorming session. What about navy moleskin pants or narrow-wale corduroys? It's usually easier for a stocky older guy to look sharp-dressed and not cliche in those than in jeans. Also, a high-end cashmere sweater wouldn't hurt.
Neiman Marcus might be worth visiting.
A medical condition technically is possible (let's include bloating -> there's information about causes thereof in the Abdominal Madness article). Loose skin is also possible. However, there's no mention of body fat percentage and no picture (which probably shouldn't be posted). The stomach is the hardest area from which to lose fat, in men. Maybe there needs to be more muscle mass to help divert calories away from the stomach.
With plain dark jeans, a plain shirt might be too drab. And a colorful one probably wouldn't look good on someone carrying extra pounds. As long as that's the case, while some party shirts and graphic tees are tasteless, might as well wade through to find some that are passable. No shiny shirts, by the way.
If that's indoors, most men probably wear dark suits. Tuxedos for weddings seem to be losing popularity. In the ideal ceremony, the groom probably should dress slightly more formally than groomsmen, as in peaked lapels and wholecut shoes.
Suspenders are not supposed to be visible. Therefore, that does not determine the shoe color. Since "black tie" was stated, even less formally dressed people will choose black shoes, and anyone who doesn't probably will stand out in a bad way.
A white pocket with a lavender trim isn't ideal, but it's close enough to solid white that no one should object.