Exact same thing happened to me. Feed him balanced spinach (this is best). Once he starts to absorb his tail he will stop eating as his body is consuming the tail. At this stage you will also want to switch his tank to a semi aquatic set up. Once his tail is gone, switch to feeding crickets.
Fish food does not have the correct nutrients they need btw. If you use lettuce, make sure it is dark like romain, but spinach is still best.