Sick tiger salamander?

Hello.

I bought my son a tiger salamander in August last year. All was well for a while. He spent lots of time above ground and was confident. He had a very good appetite and would eat crickets, meal worms, roaches, small pieces of meat and shrimps. He grew fast.

In a short period of time he switched from spending his time above ground to digging. Around the same time he lost his appetite and hasn't eaten in about six weeks. He still looks healthy when I see him and doesn't appear sick at all.

I had concerns that the substrate was a little acidic and replaced sixty percent with coco-fiber. He inspected the tank for a few hours and then disappeared. There are no obvious tunnels but I can guess approx. where he is. I'm assuming that even though there is no obvious tunnel he can still breathe.

The only ideas I have for his behavior are that it is something to do with the ph of the original substrate or that his lack of appetite is because he is constipated. No sign of any feces in months.

I doubt that the substrate is an issue now as I have changed most of it. Obviously I am concerned that he won't eat. What do you think? Any ideas? Also as he has been missing for a number of days with no sign of a tunnel what should I do? Should I assume that he is okay and leave him to it or try and dig him up?

If he doesn't eat soon I guess I will have to consult a vet. Not sure how useful that will be.

Thanks for reading.