I normally wouldn't recommend this because of thiaminase concerns, but if you have a local baitshop that sells silver-colored minnows (less likely to have thiaminase than rosie-reds at any rate) you could try feeding those until he puts on weight. They're way larger than guppies, but you could probably find them at an appropriate size for him.
Since he's only really taking live fish, it may help get him to a weight where you can afford to treat the parasites and spend time working on getting him on pinkies?

*Edit*: I just thought of this. Have you tried f/t silversides? I have one guy that absolutely will NOT take fish unless he can see the head. If one of the silversides is broken in two, he won't take the headless end. He definitely wouldn't take strips of fish.