Upside-down catfish - what can I feed him?

I'd like to vary my upside-down catfish's diet but I don't really see him eating (I know he does eat, but he's super-shy) so I'm not sure what he likes. A lot of places I've looked list insect larvae and a varied diet (not very specific). I'm trying to go through things to see if he'll eat them but I'd love some suggestions!

I took that pic of him in that container when I bought him - sorry should have explained that. As far as I know he's an upside-down catfish, he was the last one in the store (they had him for a long time). He swims upside-down all the time.
I've seen him go after frozen brine shrimp and flakes when I'm not standing in front of the tank, so I know he's eating something. I try to drop shrimp pellets but my diamond tetra loves to play with then eat them so I can't tell if he's getting to them. Maybe I'll try putting a few pellets in his favorite hiding spot, my other fish tend to stay out of there.
I keep my tank at 78, and rarely use the light on top. I have noticed his coloring has gotten darker. I check my PH a lot (and it's great) and do regular water changes so I doubt that's it. Maybe he's just shy around me? He was in the bottom tank in the store where nobody tends to look so maybe he's not used to attention like my other fish - they love to follow me whenever they can see me

Best pic I've ever been able to get of him lol. His belly is full and seems to be doing okay other then being a bit shy. I have noticed he tends to come out a bit more when I feed them flake food

I just walked by and saw him at the zucchini I put in for my pleco
And thanks for the proper ID!