Have you tried an oyster or clam (live oyster or live clam from the supermarket if you have one that sells that sort of thing, it sure is fun going up and asking for "one oyster please"
)? Or a mussel ..
I buy about 4 or 5 at a time, throw them in my sump, and about once a week I'll crack one open and throw it in the main tank.
Probably not great for water quality, but .. well, at least he's eating something this way..