Starving pionus

No shaming, please.
I've had my pionus, Charlie, for 17 years and he's always been healthy. However recently he's been getting more and more clumsy, falling off his perches, and last week, was no longer able to climb up the cage bars to his perches. Took him to the vet. The first thing the vet said when he picked him up was "He's starving to death"
I was shocked. He empties his bowl of Zupreem, and I fill it again. To be honest, I've never known how much he should be eating every day, so I didn't realize he wasn't eating enough.
My husband is the only one who can hold him so I couldn't have known he was that thin.
The vet, experienced in exotics, said to feed him anything and everything that we can get hom to eat (except avocado, onion, and garic).
Finally to my question:
I have four bowls of food in his cage, one of which is fresh food. I'm having a hard time finding things he'll eat in that category. I've tried blueberries, pineapple, lima beans, shredded chicken, hard boiled egg.
Anything you know your birds can't resist that I can try?
(The other bowls are Nutri berries, cheap Walmart seed based cockatiel food, since that what he ate at the vet's office, and Zupreem Nutri Smart.)