That is a great fish you have, I wish I could have one but don't have a big enough tank. I have a Sorubim Lima, a shovel nose but much smaller.
Mine started out only eating feeders but I have successfully trained him onto beef heart and he loves it. I go to the supermarket, buy a beef heart have them slice it deli thin and then freeze it. When it is time for feeding I break off what I need and throw it in. He usually waits until it is dark to eat it up but is getting to the point where he will now eat while I am feeding the others.
PS - They do have a big appetite. When I first got him he ate one of my Bala,s. Last night a put a new one in to replace him (a good sized one mind you) thinking he would be OK because he hasn't bothered my other two yet. Apparently he didn't like the new kid on the block, he was not there this morning. That was a good $9.00 meal.
I feed my shovelnose mostly smelt, shrimp and squid, but he will also eat other types of fish. Basicaly I just go to the seafood section in a local chinese grocery store and buy what's cheap. I find the smelt to be very economical and easy. A bag of about 50 is only $4 and that lasts a couple of months if you mix other foods in.