Hello, this is Dr. Linda. I hope I can help with your question today.
I am sorry your dog is not feeling well. There may be some issues from the surgery resulting in pain & some discharge from or around the incision site.
Hopefully, when he urinated he produced a normal stream of urine. By that I mean he did not strain to urinate & he urinated a fairly good stream & not dribbles.
If he is painful, we can safely give a little aspirin, but only if your veterinarian did not send home any other pain medications. It is very important that you follow up with your veterinarian tomorrow. I do not like to give aspirin following a surgery for any more than a day or so.
You can give him 1/2 of a regular strength aspirin ( not Tylenol or ibuprofen) per every 25 pounds now. This can be repeated in 12 hours, but please talk to your veterinarian who did the surgery first.
As for not eating, offer him something special a few hours after the aspirin. Hopefully, if he is more comfortable, he will be more willing to eat.
I hope this makes sense & you find the information helpful. Please let me know if you have an additional questions or concerns.