Guppies are better than goldfish feeders, and are good for establishing a feeding response from your lion. Now that a response is there, I'd recommend feeding the lion very well for a week, make sure he gets plenty to eat, and then don't feed him for a week. At the end of that second week, he should be ready to eat. He established his feeding response in week one, recognized you as the feeder, and then in the second week he's geting hungrier w/ no food, and he'll start looking.
Then you can easily feed him w/ a feeding stick. Or you could get lucky as I have w/ most of my lions and they'll eat the frozen krill/silversides or whatever as soon as it hits the surface of the water, as the powerhead moves it a bit to get their interest.
I used this method for every lion of the dozen or so I've owned, and I have never had an issue getting any of them to eat. Good luck. And don't worry about the guppies, LFSs always push them, and they're not harmful in the short term. Your lion won't wake up dead tomorrow w/ a defunct liver. But I would wean him off the guppy fry over the next few weeks/months.