Give him a week or so of fasting, no food at all; he's big enough not to need food every day. Then, when you think he's good and hungry, offer some food and just sit in front of the tank quietly and watch him. If he doesn't go for it while you're there, remove it and try again a couple days later. He's not going to starve to death. He will eat. If you cave in and leave the room with food in the tank, then you have just wasted that week. It shouldn't take long before he's associating you with food. You can then go to using tongs if you wish.
You know what they say: you can't teach an old puffer new tricks...or something like that.

The fact that he's gone this long being allowed to behave like a prima donna adds to the challenge, but he can be trained; it'll just take more time and effort.
Training a fish (or a dog, or an apprentice or whatever...) doesn't consist of simply staring at him and hoping he will read your mind and do what you wish. You must make him want to do what you want him to do. You must outsmart him.
In this case, it shouldn't be that difficult; he's just a fish...a
thinking fish, maybe...but still just a fish.