If he was attacked by other cockatiels in the store it may be that your other bird’s presence (in addition to yours) is stressing him out.
Can you move him to a quiet, low traffic room (like a bedroom or spare room) away from your other bird and just let him decompress? Like, just keep his cage clean and his food and water changed but give him space. I’d give him a week of that before slowly starting over with introducing yourself.
When I say slow, I mean, grab a chair, walk as far into the room as you can without him panicking (go very slowly, watch his body - you want to stop while he’s calm but starting to get a little alert), sit in the chair, and IGNORE HIM. Don’t look at him, don’t make noise. Read a book or something else quiet. Do this for about 15 minutes and then leave. Mark on the floor with tape where you were. He may become more or less tolerant of how close you are - you may even find yourself starting outside the door! Just make sure he can see you but is NOT panicking, and do it until he seems relaxed (I find using my phone’s camera is a ridiculously handy way to keep an eye on a bird without looking at them directly! But don’t point the camera at them - kind of angle it so they’re just in view).
This is gonna take a good long while. Again, keep him away from your other bird because that sounds like it’s a major stress trigger for him.
Eventually your goal is to be sitting next to his cage. Once you are there, the next thing you want to try is to put your hand on the table/wall/counter where he can see it, below his eye level and far enough away that he’s not freaking out, and just leave it there for a couple minutes. Slowly remove your hand. Repeat until he’s relaxing - grooming himself, doing other things, whatever.
Once you can put your hand next to (BUT NOT ON) his cage, THEN you can begin offering treats. Use millet, below eye level, don’t look at him, and make sure it’s a long piece so your hand is very far away, and close it into a fist so there aren’t visible fingers (no idea why but I have found with older store cockatiels they really hate fingers but can deal with a closed fist). Wait. As long as he’s not panicking, even if he’s on the other side of the cage, he may come over to check it out. Give it about 30 secs then remove, wait a couple minutes, repeat. If he eats some, slowly move your hand back (he will need time to eat and if you move your hand away when he does you’re showing him that he gets a reward and then the scary thing backs off, so it’s a positive experience).
If at any point during this process he panic flies away from you - start over. Go back a step or two, as far as you need to, for him to remain alert but calm. It WILL WORK eventually. It may take a really long time! Cockatiels are ridiculously social though, and his need for socialization will win out against his fear if you are patient and respectful of his boundaries.
Cockatiels are lovely little birds who are often overlooked and I thank and respect you for giving this fellow a chance. It may simply be that he’s old enough that on top of being afraid of cage mates, he’s hit puberty which is making him hormonal and even more stressed.
no news yet we try everyday but probably it’s too soon as you guys told me I will keep you update
