Young pigeon can’t fly

Hi. I’m hoping someone can help please? 
I rescued a pigeon from my neighbours house yesterday after her cat had ‘bought her a present’

i couldn’t see any injuries but didn’t want to stress him so didn’t handle him much. I put him in our cat basket in the airing cupboard. I gave him an electrolyte solution and some wild bird seed and left the poor thing in peace. I didn’t expect him to make it thru the night but got a pleasant surprise this morning when it was alive and kicking! 
I tried to release him in the garden. He will walk around on our kitchen storage box but won’t fly off. His mum or dad came down and fed him. He flexes his wings and flaps but can’t lift off. I spent an hour and a half guarding him (from cats - next doors cat made an appearance and freaked him out again. We managed to catch him again (he had hopped off the box and hid behind a bush). I think he sees the cat basket as a ‘safe haven’. We left him in our green house (which isn’t too warm now)  to calm down and rest again. We tried to release him again. Mum came and fed him again and he tried to fly but ended up on the floor. We’ve caught him again so he’s safe for tonight. Not sure what else we can do? Can I put him up in the very large fur trees (where I think he came from) in our neighbours garden? - lots of pigeons are in the trees. 
he’s about two thirds the size of his mum and has mostly adult feathers but some fluffy ones as well. He fluffs up and preens himself. He’s eating and drinking - some from his mum twice today and seed from us. Our garden is cat city and I don’t want to leave him as cat takeaway. Any advise greatly appreciated. Thank you