You don't need to do anything
If all of your players could fly then you would need to build your encounters accordingly. If you only have one flying player then they will naturally balance out. The key advantages of flying are:
Getting around cover
Avoiding melee with grounded enemies
Increased mobility is helpful but, unless your ...
First, a note on dealing with the Aarokocra - don't do the following tips in every fight. Every player wants the chance to shine so let them have it. The ability to fly around the battlefield unimpeded is a lot of fun so remember to sometimes let them do just that. Create scenarios where their ability to fly is instrumental to a quick, clean victory. But don'...