If I understand it right, he ate properly when you got him? But things changed when you changed his tank? Or has he been picky from the start?
I would always suspect IPs when a puffer is new in the house. Most puffers are wild caught, and will bring a load of parasites. The stress they've been through when getting caught, packed, shipped, packed again, shipped again, and so on, until they end up in our tanks, is enormous, and the puffers often get ill when they come to their new home. Their immune system is lowered because of the stress they've been through, and then the body is less fit to control the parasites. I always take wild caught fishes through a deworming when I get them into my house (after a few days of letting them get to know their new home and lowering their stress level - medication can be a stress factor in itself).
Have you tried feeding him shrimp/prawn? That's always been the way to success when I've had trouble with puffers getting started eating in their new home. If you've had trouble making him eat from the start, it could be that he doesn't recognize the food you feed him as real food. If that's the problem, you have to trick him into believing dead food is alive (a shrimp/prawn in a thread will do the trick).
But if he did eat well when you got him, I would suspect IPs being the problem.
But most of all, I suspect you are having high nitrates in your tank. 60 litres on a single irrubesco is already pushing it - it will work when the puffer still is small, but you have to have very good routines for water changes. But 60 litres for a puffer
and all the rest of the inhabitants is way too much. It's not an easy thing knowing that it can be the problem, as you probably don't see the other fishes reacting on the nitrate level. But puffers are much more sensitive to nitrates than most other fishes, so any high level will affect the fish.
And a few more questions before I finish this long speach

Do you soak the dried food and thaw the frozen food before feeding? If you don't, it can give digestion problems for your puffer.