Firstly, it's wonderful that you cared for this little guy and did all you could to rehome him.
There are a few things to take into account.
You don't know this little guys background- whether he was from an aviary, or was a solo bird, but giving a solo bird makes them bond to their mirror. They see another bird, think it *is* another bird and want to hang out with it.
They are a social creature and he would like to hang out with the bird that lives in his cage.
So step one, remove the mirror.
You said when he's tamed you want to learn to give him veg but that's a really good healthy part of his diet, so there's no need to wait. start trying with some wet leafy lettuce, like romaine and he'll *love* it.
The taming and bonding section will give you step by step advice os bonding but you should start completely from scratch. Use his body language to help you determine what he is comfortable with.
Start by sitting with him, talking or singing to him and go from there. You can give him a month or two and give him the chance to adjust to a bond with you.
If you decide to get another friend, a boy will be best (my flock of 8 females and 6 males has stronger bonds between males over males and females) but you have to consider that they might not get one and can you house two cages on a permanent basis. That's a worse case scenario, but you need to consider it still.