Being burrowers it may well be that he has gotten some bedding stuck in the corner of his mouth or possible further down.

Open his mouth and look, maybe even give him a flushing under the faucet.

I'd also move him off the walnut and put something that is larger for him to burrow in. Pine shavings or aspen have worked fine for me for many years. I've never used walnut or coconut but I have used sand. I like the wood beddings the best.