Blackbird rehab.

I found what looks like a blackbird (according to google lens) cowering in the corner of my garden. I left well alone as it's quite a secure walled garden but the next day it was in the middle of the lawn being pawed by my dog out of curiosity and I thought it was dead. When I went to pick it up it was still alive albeit a bit scraggy. I put it in our  covered tortoise run where it's been for a few days now and I've been feeding it cherries from the tree and a few ground up almond nuts. (Dog food just attracted hundreds of flies and ants and the bird ignored it anyway). It's really perky now after a few days and is hopping about like mad but when I lift the lid off the run it can't quite seem to make it out to the top (about 20 inches high). I don't know if it's because of injury or if it just hasn't learned to fly yet. Should I just put it back into the garden now (and keep the dog out) or wait until it can fly out of the run by itself?  When I was a kid I remember my brother rescuing birds and he used to put them up high to encourage them to fly off but I don't know if I should do that in case it just can't manage it yet. 

I'm going to get some proper bird seed or sunflower seeds from the supermarket today to supplement the diet of cherries and nuts. Oh yes that's another thing - I noticed it seemed to be swallowing cherries whole so I've been taking out the pips which is a bit of a pain. Is that necessary?  My tortoise run has a compost base and has been deliberately left to grow full of natural weeds so it's not a bad habitat for finding insects so hopefully the bird is foraging as well.  

Any advice welcome - I know nothing about birds as you can probably tell.