Hello, I'm Dr. Deb.
I recently came online and see that your question hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
I can understand why you might think that one or both of the two medications that your cat is taking might cause constipation issues but this symptom wouldn't be an expected side effect from these drugs. In fact, side effects from either one of these drugs usually involves looser stools so this is unlikely to be the cause of him not producing a bowel movement for 3 days (if he hasn't already produced one).
Either or both of these drugs can affect the appetite, however, so they could be responsible for his disinterest in food.
You might ask your vet about appetite stimulants which might help. Mirtazapine or Cyproheptadine are two which I've used in the past with some success (although not all cats respond to these drugs).
It's possible that anesthetic drugs during his surgical procedure have caused this problem or he's wary of producing a stool for some reason.
There are options which can act as laxatives and which can be added to the food you're currently syringe-feeding him:
1. Hairball laxatives such as Laxatone or Laxairewhich are brand names but every pet or grain store will carry these products. I usually suggest 1-2 inches 2-3 times a day.
2. Canned pumpkin (NOT the pie mix) at a dose of 1/4-1/2 tsp per day.
3. Over the counter, human Miralax at a dose of 1/8th-1/4th tsp daily which can be increased until soft stools are produced.
It's possible that he'll need an enema but this will best be administered by your vet.
Since he's not consuming as much food as he normally does, I probably wouldn't be too concerned that no feces has been produced for three days, but I would expect him to have a movement today or tomorrow at the latest. If the laxatives are unsuccessful and he still hasn't produced anything by tomorrow, then a vet visit may be prudent.
I hope this helps although, again, my apologies for the delayed reply. Deb