The vendor has him for over a month eating so @Jay Hemdal thoughts?
Are you sure you are feeding the same, or similar food that the vendor was?
The puffer could be intimidating it, I've never kept those two species together before, so I don't have a clue how they would get along.
I dimly recall having good success feeding baby guppies to one of these, but I had access to hundreds of guppies.....
Jay