I think he should lean dragon pulse, earthquake, flamethrower, and fly. these all do more then 90 damage so that's good. Plus earthquake is good to use against rock and electric types, flamethrower is good on ice Pokemon. and dragon pulse is good against dragon. These types are all super effective to you so its good to teach then attacks to use on them.
Or you could teach him
dragon claw/outrage
fire-blast/blizzard
surf/thunder
extremespeed
Rayquaza's best move is Outrage when you see lots of fire balls comeout
icebeam is super effective
rayquaza is he can learn every move
it could be useful why
he doesnt you have to teach him it
i think that hyper beam is a pretty good TM to give rayquaza. and keep outrage if you hav'NT already deleted that move.
Teach a Pokemon fly then go to Raquaza then use the move fly on him.Also have a friend bow to recruit him.
This means that your Pokemon probably has learned fly. This move is not allowed to be transfered from game to game by migrating. You should take rayquaza to the move deleter and have fly deleted and then you can migrate it to diamond and teach it fly then.
Roar of time.
The best Pokemon to catch in emerald is rayquaza. It has great speed and awesome strength. If you teach it the right it might the strongest Pokemon.
final blowup
i think rayquaza because you need both groudon and kyogre to get rayquaza