Have you tried different foods? Maybe he wasn't fed flakes, and isn't used to them. Sometimes I'll get a new fish that isn't used to what I'm feeding on a particular day, and it won't recognize it as food, so either ignores it or spits it out. Sometimes they do spit out the food if the piece is too big (Rainbows have small throats) and then re-eat it as it gets smaller.
I would try brine shrimp, bloodworms or Tubifex, and maybe try some pellets. Or even a mix of all three--he's bound to like one of them! Cut up the bloodworms to make it easier for him to eat them. I have noticed a lot of fish don't seem to have been fed flakes, but will go for the pellets, or vice versa. Also, what brand of flakes are you using? Maybe he was used to another brand. Try something high quality, like Omega One, or New Life Spectrum.
I would try brine shrimp, bloodworms or Tubifex, and maybe try some pellets. Or even a mix of all three--he's bound to like one of them! Cut up the bloodworms to make it easier for him to eat them. I have noticed a lot of fish don't seem to have been fed flakes, but will go for the pellets, or vice versa. Also, what brand of flakes are you using? Maybe he was used to another brand. Try something high quality, like Omega One, or New Life Spectrum.