I'd just like to add a comment of what I've personally experienced. I do have a worm farm, but because my guys are so ravenous they nearly wiped it out. I got alternative food (pellets) and I've noticed a substantial increase in growth (with both kinds). I think it's like dog food ingredients -> chicken (80% water) vs chicken meal. Since the chicken meal has the water removed, it is highly concentrated in proteins. I've feed them less volume-wise with pellets and yet they started to grow faster. So perhaps while he's recovering, it would be better to feed a good quality pellet food?
This was just leading up to: perhaps try a better pellet? From what could find out, soy bean meal is the first ingredient in yours. I can't find which specific food one it is but if it's the one I think it is the protein content is around 35% which I would consider being too low. (feel free to take a picture of the ingredients list and guaranteed analysis!).
I would personally vouch for the TetraCichlid jumbo carnivore sticks I found at Walmart (not sure if you have that in the UK?), which have 47% protein, and shrimp meal as first ingredient. They float naturally so I generally crush them with my fingers first to let out air, then finger feed them to my axies. If you can get a hold of Rangen soft-moist salmon pellets though, those are pure gold - my axies absolutely love them! They'll probably eat a staple of Rangen, but have worms as treats throughout the week.