First, if the diarrhea started that late post surgery, we'd not assume it was directly related. Instead, I'd be most suspicious that the stress/energy loss that the body has undergone with surgery and healing has allowed an opportunistic bacterial GI bug to gain a foothold here. So, while it is good that worms have been ruled out and the initial supportive care (which was more geared to treat this like a dietary indiscretion or imbalance in the good gut bacteria post surgery), it sounds like this is not going to be enough. Especially because if this was one of those 2 benign issues, we'd expect him to be responding already.
Since he isn't, I'd advise ringing his vet and updating them on this situation. You can request (since they did just see him for this) that they dispense a broad spectrum antibiotic to tackle any lurking bacterial gastroenteritis. Alternatively, you could also submit a fecal sample for bacterial culture. Though we'd likely still want to use a broad spectrum treatment since cultures can take a while to get results.
Overall, if this was just a benign GI upset, I'd expect it to have already responded to your good supportive are there. Since it has not, I would advise being proactive at this stage and ringing his vet about antibiotics +/- full fecal testing to ensure get this settled as quickly as possible.
All the best to you both,