Giardia or other unknown problems?

History: My chinchilla, Taro, is 2 years old. 4 months ago, he wasn't eating, crushing his pellets, drooling, pawing at the mouth and in pain. We took him to 3 vets, got xrays, the vet found spurs on his mouth. He came home and ate CC from me, a couple days later ate on his own and things were good until...

2 weeks ago I noticed he was having problem eating his pellets correctly (crumbling), hesitated to chew his apple sticks, was losing weight (20grams). I started feeding him CC. I immediately took him to the vet assuming it was a teeth problem. He got his teeth trimmed. Vet said she found an ulcer in his mouth, minor spur which she trimmed. He was given metacam and bactrim (TMS). Came home, he refuses to eat (even CC which he usually loves). I waited a week and he still wouldn't eat, but would take treats (like the horse cookies) and he chews his apple sticks just fine. He lost 10 more grams, so I took him into the vet again.

We got xrays and his feces analyzed. His xrays looks good, if not great. No problem with his roots, his teeth were aligned (I will post up the pictures soon). His feces came back positive for Giardia, but his symptoms (no appetite, very very small poop that are hard) don't seem like Giardia. I'm skeptical as to whether the analysis was incorrect of there was an error. He still eats very very little (a little bit of hay, 5 pellets or so a day) on his own so I've been force feeding him. The vet said to temporarily stop his medications to see if anything changes since the meds might be giving him side effects. The vet is sending me medicine for Giardia for him and my other two chins (since it's contagious) who are separated from Taro (we partitioned a ferret nation in half).

My question is, although his symptoms don't seem like Giardia - small poop, not as much quantity as before and no appetite, could it still be Giardia?
Given that his xrays for his teeth are good although he had teeth issues before, he still isn't eating. Could there be another underlying problem we missed?

Here are the xrays taken.

From what vet and I can see, his teeth are good. Although I'm no expert and haven't seen that many xrays to comment.

I also noticed two circular empty spots on his body xray. The vet said it is gas. Is this problematic? I haven't seen any other chins with the same spots.

Is he pawing or drooling this time? The x-rays don't look bad to me but they are a little fuzzy, may be my moniter. Problem with x-rays is until a soft tissue issue gets really bad it does not show up, things like abscesses and periodontal disease. The giardia can cause appetite issues but the giardia could have surfaced due to stress on the body from the mouth. If it was my chin, I would treat the giardia and see if that helps, if not I would ask for a blood panel to rule out issues such as liver and infections. The gas can be a byproduct of the giardia, I would do simethicone during the treatment.

ticklechin, where would i get simethicone from? the vet said to give him tums (the one used for babies). is that safe?

nevermind i found out that i can get it at the drugstore. how much is safe for dosing? he is a fairly young chin and weighs 405 grams now.

Simethicone stays in the digestive tract and is not absorbed, it can't really be overdosed. I give a full dropper every 3-4 hours.