Porcupine Puffer is struggling to eat

I have recently had a few problems with my tank with a high Nitrate spike which I have now sorted but this led to most of my fish getting ICH. My Porcupine Puffer wasn't really interested in eating while he was infected but now when he tries to eat the muscles I give him, he breaks the food up but can't seem to swallow it and it all comes back out.

I have tried feeding him whole muscles and clams in the shell as I read that they need to break their teeth down now and then as they continuously grow, but he has not taken to trying to break them open.

Can anyone offer any help and advice on what I can try to help him eat again?

Thanks