Will Mega Rayquaza will go back to its normal form if the opponent has more Pokemon?
If Rayquaza forgets Dragon Ascent in battle, and there are more Pokemon left, it will stay Mega Evolved til the end of the battle, but can't Mega Evolve in the next battle. Hope this helps. ;)
Source: I Tested it.
You can reteach it to him, because there is a lady in meteor falls that will.
