Be better off not feeding feeder fish to him. Feeders can carry diseases that could kill him. Breed your own feeders if you absolutely have to feed him those, but their are much better choices to feed.
MY 4" PB's get frozen bloodworms and I'm also giving them small live worms every couple days. Eventually I will transition them to pellets (hopefully), but not yet.
I've tried frozen bloodworm cubes but if anything he'll bite it, let it go and go on with his business. Now he will eat the frozen bloodworms off the bottom of the tank but other than that he won't eat anything else.
If he mouth's it then he is interested he just dosen't know what it is yet. Try cut up talapia/shrimp/Pellet's soaked a little/freeze dried krill soaked a little pretty much just keep trying everything this is the time to train him/her to eat what you want to feed them.
When you feed feeder's how do you do it? If you don't mind me asking.
Getting him on blood worms should be pretty easy. After that its pretty smooth sailing. I have some wc monos about that size and they LOVE frozen krill. I just have to tear it in small chunks.
I've tried frozen bloodworm cubes but if anything he'll bite it, let it go and go on with his business. Now he will eat the frozen bloodworms off the bottom of the tank but other than that he won't eat anything else.
You thawing the bloodworms at all? Put the cube in a glass of water, wait like 5-10 min. Shake/stir it up so that the bloodworms separate and then toss em in there. He will start hitting the individual bloodworms.