I would keep him on live until he's over 100 grams. Small frequent meals. Either hopper mice or rat pinks if he will take them. He's smaller in weight than most of the BPs I hatch. I wouldn't bother trying to switch him to FT or prekilled until he's eating very regularly for you.

I don't know why the breeder would have allowed him to stay this way. He should have fed him live if that's what he wanted. Sometimes it takes a few months and some growing to kick in the feeding response strong enough to really begin having any success with FT switching. Until you get to that point why allow the baby to practically starve?