I've never owned a Pacman (yet), but I've read enough about them to help you. I certainly can't explain why your frog is spitting out whatever is offered, but I can ask you some questions, the answers of which might provide some insight as to why he might be doing that. Also, for the record, many exotic animals bought at chains like Petco are often in less than ideal health to begin with.

What are your temperatures/humidity levels?
What substrate?
Do you use dechlorinated/treated water for ANYTHING the frog lives on?
What are you attempting to feed him?

That's a start anyway.