Any/all ideas for cat stimulation please

My cat is a 3 year old domestic shorthair. Normal weight, healthy, gets a lot of attention and care, but he is just always begging for food. The shelter doesnt know where he was since he was born, but they found him on the street in their town. They assumed he must've been somebody's cat as he is incredibly calm and docile. Not afraid of anyone, immediately let me pet his belly, pick him up, pretty much do anything and everything without a care in the world. The problem is that he only sleeps and begs for food. Think of any toy or place to hide/lay for a cat and I got it for him, but the problem is he showed interest for anywhere from 5 minutes to an hour and then it's never touched again. The only thing he consistently likes is a 3 foot tall box laid on its side with packing paper in it, but all he does with it is sleep in it. I've tried cat tv, toys, playing with him, perches, catnip, hiding spots, and pretty much everything else but he just doesnt care. He only ever wants food. Any ideas as to what I can do or get for him cause I dont want him to be depressed or anything.

Tldr: healthy, calm cat has everything i can think of, tried all i know in terms of stimulation, but all cat wants is food and to sleep. Any ideas what i can do/get?