Cyn is right..pick him up, grab his beak and hold it close for a minute or so and tell him "NO" firmly and repeat it until he stops struggling...release his beak and if he tries to bite again, hold his beak shut again and keep that up. He will get the idea (hopefully)...not every roo can be fixed that way but he is young enough you can stop it.
Fattie thrashing around like an animal enraged when I hold her beak closed...I hold it until she stays still. She no longer bites my hand when I reach in, she just nips to get my attention and get picked up or patted.