Oh, I was imagining he was an old horse, not one who's hardly middle aged. It sounds like lack of exercise is at least part of the prob. I would be trying to exercise him as much as you can manage. Even if it's only taking him out for a half hr walk twice a day, that would help a lot.
Agree with Appy that a joint supp is probably not a bad idea. Also agree that asking advice from a *good* vet would be prudent. If he has the beginnings of arthritis, you might also ask the vet about Cartrophin(sp?) injections.
If he's only getting what you've told, he'll need a good general supplement, as he's likely deficient in a range of nutrients. It depends what's in the hay & seeds as to what he'll need, so I would be getting a basic analysis done. I'm no nutritionist, and I use a very good(& economical... & independent of feed co's) program called feedxl.com which makes balancing diets easy. A nutritionist may be able to tell you of specific supps that would help too.
Why do you feed oats? Is he in hard work & needs extra energy? While oats are about the safest cereal grain for horses, all grain is potentially problematic and I would be avoiding it unless necessary & no better alternative. I would only feed grain if I could give the ration in at least 3 feeds per day too, rather than just one.
Yes, standing f/t in mud is not good for hooves & can reduce hoof function & therefore circulation & can lead to thrush, which can make heels very sensitive. But IF his feet are in a good way & it's only short term it's not likely to have too much effect, and this sort of sensitivity would not be something that would get better with exercise. Is he shod? If you would like an evaluation of his feet, send some hoof pics, pref just after a trim, clean feet. Front- & side-on from near ground level, and a few different angles of sole, including sighting down from heels on slight angle and side-on on slight angle, to get an idea of depth.