Are you feeding the critical care with a fairly huge syringe? The one I used was almost the size of my thumb and I used to add extra water to more than the bag suggested so it would flow more freely. You have to get it down him whether he fights or not. I used to prop my little Ernie between my knees and put my feet on a coffee table so he was sitting almost straight up. I put him in a towel (looked like a little taco) and did leave his front feet out but he couldn't move too much and put the syringe a little ways into his mouth and pushed a little critical care in, just a little at a time. I gave him water with a tiny syringe in between the critical care to wash it down and he soon learned to love the water from the syringe. I know it can be tough but you can do it and so can he. My little piggie had malocclusion problems for 4 years and had his teeth done 6 times during that time and I learned to be quite the expert with the critical care and each feeding took me almost half an hour. The vet should be able to tell you how much to give him according to his weight and it is important that you find this part out. I fed Ernie every 8 hours and I did end up feeding him at 4 in the morning for a week or so but soon he began eating on his own. Sometimes after surgery he was eating well a couple of days later and other times it took a week so it can be a little bit of a challenge. If they give him meds that can also sometimes play with his appetite and another reason he shouldn't have much of a choice, grin! Just be persistent and one way or another he has to eat and even though they sometimes don't like it much, the critical care will sustain him and give him the energy he needs to get better. Guinea pigs don't retain water for very long so my little Ernie got a drink from the syringe everytime I went by the cage. The syringe was tiny and the water came out in drops and he got 2 or 3 syringes full of water but it worked. Please keep us updated and I hope some of my experience has helped. PS I had to use a toothprick to unclog the syringe sometimes with the critcal care so you may want to keep one handy. Piggie huggsssss.