Leopard gecko's remarkable lethargy and weight loss during recent 3 weeks

My leopard gecko stopped eating probably about 3 weeks ago. He has lost a significant amount of weight and is remarkably lethargic. He is now 29 grams and a little over a year old. My gecko never liked crickets. When he stopped eating we tried wax worms and super worms and he refused to open his mouth to eat.

We took him to my mom’s work (vet clinic) and the doctor said he has sores in his mouth, which we are assuming to be the reason he won’t eat. We took him today and they said tube feed him twice a day. We got all the ingredients and everything for the mixture. When we brought him home he is laying flat on his stomach. He is breathing and his head is lifted up. I was holding him he moved a few times.

I’m just really worried. His eyes have been sunken in for weeks now. We’ve tried to feed him something different and he won’t take anything. The sunken eyes started after he had shedding stuck in his eyes. The vet got it out and he still wouldn’t open his eyes.

Is there anything I could do to get him more healthy as soon as possible? I’m scared he won’t make it to the next meal. I feel extremely upset at myself for not taking him to the vet sooner, but the vet was always booked.