Mean beardie

Hi everyone! It’s my first time posting on here and I was wondering if I could get some help.
So from a pet store I work at, I got a beardie. He used to be really nice just like all the others, but once another beardie bit his tail and the end had to be removed slightly, he changed. I decided to take him home to keep and take care of him because I already have a beardie, so my job knew I’d take care of him BUT ever since he has come home he is SO mean! He has bitten my dad multiple times even though my dad makes sure to hold him every day while I’m at school. It’s been over a month and I see no improvement. Now I’m getting nervous because he’s starting to get bigger (he’s like 6 inches long or so) and he’s getting stronger to the point where I really don’t want to be bitten now.

When we try to hold him, he’ll try to back away or he’ll just straight away lunge at you and attack

I have no idea what to do at this point and I’d love some feedback! I don’t want to leave this crazy mean lizard with my dad to deal with himself when I go off to college.
Thank you!

The worst thing a fully grown adult bearded dragon could ever do is maybe draw a tiny bit of blood. Out of all my animals i have i rather be bitten by my beardies or even my tegu rather than my juvenile blue tongue skink.

At a young age he's most likely just terrified of his new environment and the other strange larger animals around him (being us and other pets), take it slow. Hand feed him daily by tongs, talk to him, dont try to touch him unless he allows it somewhat. However once he calms down more and is bigger and more hold-able you can handle him, but keep him down low to the ground in case he jumps.
Having his tail bitten most likely caused a lot of stress so he's nervous of you and doesn't trust you yet, you need to earn his trust.

He'll eventually calm down however this will take days, weeks, months or even years in some cases.

This isn't what I did with my beardie, but with my leopard gecko. She was living at a very ill-informed person's house, and needed a lot of help. She was shy and scared.

I started by just putting my hand in the tank for a while. Eventually she'd come over and lick it or check it out. I did this for a couple days, and then she eventually walked on. The next time she did this, I took her out for a couple minutes. And then a couple minutes longer each time. Now she's sociable and happy. That may work for you, but your mileage may vary.