This would be my plan if I were in your shoes.
Don't feed him until he accepts live guppies. He needs to be hungry to train. If he's being that selective, he's not hungry enough to be motivated. The guppies are important on a couple of points in that they are not too big and their silver bodies make for an easier transition to something like silversides. You can't train him by using a large meal. Liken it to when you train a dog using small treats for reinforcement.
The 50lb fishing line allows for enough control that it is like a more flexible, invisible extension of the stick and not a flimsy thread on the end. Sometimes I was there holding food until my arm when numb and jiggled from exhaustion. It can be a lot of work and take upward of 30 minutes a session to get them to go over. I'll get you videos or pictures or whatever you need when you get to that point, to demonstrate some techniques.
First things first, I would hand pick out some of the larger guppies at the LFS and withhold food until he's hungry enough to forget about Roseys.