Is he breathing fast? If so, treat for ich or flukes.
If not, you could try pieces of clams. I add selcon or vitamins to the pieces. Once he eats aggressively, you mush a piece of the food you want him to eat with clam pieces (with your finger) and feed him. After he eats that aggressively, let him starve one day. Then feed only the food you intend. I personally used NLS pellets.
This has worked for all of my butterflies that were picky!