He's hungry... feed him!
My orange spot rabbitfish has grown from 2.5" to nearly 6" in about 8 months, outpacing my Scopas tang by double. Juvenile rabitfish grow... and to do so, they need to eat.
I feed a half sheet of Nori every day, a quarter sheet in the AM, and a quarter sheet around 5, with a variety of more meaty foods around 7pm.
IMHO, the 'low nutrient' mantra, blaming high nutrients for algae growth, reducing feedings to an absolute minimum, has led to many of us underfeeding our juvenile fishes. I like my fishes fat and happy. Much less aggressive that way, and less likely to opportunistically feed on other reef inhabitants.