that's good news
i think it's the same as mae, she was undernourished too and all her problems pretty much stem from that.
it is important that he eats proper food for his teeth but in the meantime it's important he has the right nutrition gets the right amount of food. mae was on critical care for a long time and i did worry about it.
i'd recommend you weigh him at the same time each day. get a scale that weighs in grams - i have a postal scale and find it helpful to stick them in a box to do the weigh in to keep them on the scale and reasonably steady. mine are ok without the lid but you can always stick them in a carrier or something like that. few grams lost is no big deal and you will find that he may lose a little bit of weight when he starts pooing properly so don't panic if it's only like 10g.
you will know how he's doing, his energy levels and if he's chewing on other stuff. try to hand feed him nuggets or pieces of hay, he may take that from you and start nibbling on that.
i did wean mae off critical care rather than just stopping it because she was on it for such a long time and she had a lot of problems. so i just increased times between feedings and gradually stopped, making sure her weight wasn't going down dramatically - it took about a week. some chins also get very used to being hand fed and mae is starved for attention. so i still handfeed her pellets a couple times a day. just hand one over to her, wait til she eats it, give her another one. she eats in her own time as well but she's still getting the same sort of attention from me as when she was being handfed all the time. i also thought it may help her understand that this is what she needs to eat now.
also, you may find that he will eat critical care off your finger. it's very palatable (not to me, i tasted it once, it's rank!) and chinchi ate his straight off my finger (in fact, he bit too hard so i fed him off a spoon, just made sure he couldn't bite into the metal). mae didn't like the taste of hay to start with so it was syringed first, then she was eating it off pellets i held for her (they're half moon shaped so worked like a scoop) and in the end she just licked it off my finger. i found it better as there's less risk of them gulping air that way.
standard critical care clogs up a syringe regardless of how watery you make it. you can grind it in a coffee grinder - i bought one when mae didn't eat it and had to wait for the fine grind delivery. seven 1-minute grindings makes it fine enough for a syringe.
fine grind is great but it only comes in a small pouch so get a couple if you have to use that but i'm sure the vets will tell you what they've been using.
if you have to syringe feed, 2.5 ml syringes are best. make the critical care as thick as you can but thin enough to get it into the syringe easily. it does get thicker with time (quite quickly) so you may have to add more water after filling a few syringes. i have a little mug, i mix it, keep the hot water running, fill a syringe, stick it under water to wash off the outside that's covered in critical care and repeat and repeat and repeat. they for some reason don't like the critical care to be on the outside of the syringe.
critical care lasts 24 hours after being mixed but i did find that mae didn't want it unless it had been in the syringe rather than in a cup and sometimes she refused to eat it unless it was freshly mixed. she definitely liked it most when it was fresh, maybe because it's warm.
if you feed off a finger, a 30 ml syringe is great, you can syringe small amounts onto your finger. the pressure is higher in this syringe but it doesn't matter and that's why i don't think it's suitable for feeding but as you're just squirting onto your finger, it doesn't matter.
if you feed off your finger with standard critical care, mine liked it made into a paste rather than liquid but it had to be rather moist - you will work out what yours likes best
also, this may be of interest:
http://chocolatechinchillas.com/id6.htmli ordered this for mae after someone on the forum recommended it, dawnna was really great and express shipped it for me. it's got all sorts of good stuff, prebiotics and probiotics and it stimulates appetite. i think it tastes nice as well because when i sprinkle it onto pellets, they eat those first. it's very good for use with antibiotics as well - i'm giving it to the boys now and thy have had no change in poo at all.