The large superworms do take a while for smaller frog/toads to pass. I know when I have super worms for my cane toads, and drop a few to my American toads it does plump them up for a few weeks.

The only guess is that the shells take longer to move through them, but I've never had any actual impaction problems. Just don't go crazy with feeding him, let him decide when his appiete returns.