It doesn't look like bumblefoot to me. The feet seem swollen, but that could be from water retention.
His internal organs are probably very stressed right now - he could be retaining water. He is on antibiotics and that can be hard on the stomach, as well.
I wish this were more straightforward, but he could have congestive heart failure, could have been bitten or stung by something and had a bad reaction, could have reproductive system issues, an antibiotic-resistant strain of bacteria in the bloodstream, cancer.......
As long as he is willing to hang in there, I would try to help him, but without knowing what's going on through blood tests or a vet exam or at least a fecal test, we can only suggest possibilities.
A fecal test is often relatively inexpensive - perhaps you can get that done.