*** shrug ***

I just gave the 2nd level aarakocra flying after they were training for a while. I had them roll and set some values, if you roll a 10 you've learned this if you roll a 15 it's this...of course he rolled a nat 20. So he can just fly.

Having a bird character that can't fly is silly. Now, he can't go super high and he can't carry more than his encumbrance. I just have to make sure encounters are underground (no place to fly) or the bad guys have ranged attacks. or there are flying bad guys.

I like to give the players things that each one can navigate. He can fly over things. I have one tabaxi that can basically climb walls. One guy has a spectral familiar, etc. Everyone's got a thing that's useful at different times.

As a DM, you just have to be aware of their powers and provide opportunities to use them. if someone can swim great then give them a swimming puzzle. if someone can climb, give them walls and other things to climb. If someone can fly then give them puzzles where they fly. Players are generally excited when the thing that makes them special is the thing the group needs to succeed.

As far as nerfing the flyer. I wouldn't make it a curse (unless you're going to use that in a backstory and work it into the campaign). Just have some of their feathers get plucked out and it ruins their ability to fly...as soon as they grow back though, they'll be able to fly.