nothing works better than live bloodwrosm or garden worms.
Eels are very shy and may not take frozen food at all. In my experience they have never shown any interest in prepared food like flake, so i wouldnt bother for now with flake.
Try turning the lights off and leaving him alone with some worms and watch what he does.
Eels do go on hunger strikes when they first arrive in your tanks and may not eat for 2 weeks, which isnt healthy, this can lead to death, but im sure if your using bloodworms, even though they are frozen, he will come back out and eat.
If you watch him closely at feeding time, he should show a sharp forward movement towards his "prey" or in this case the worms. You should be able to recognise feeding when u see it.
Goodluck, i think there is a profile of a spiny eel in the fish section or under oddballs, im pretty sure its in fish profiles under odballs, ill find a link in a tick.