Firstly, what training HAS this horse had? If none aside from leading, or if you have no idea, agree that I'd start him from scratch. I'm guessing from your words, you don't have the experience to do this yourself, so employing a trainer would be a very good move. If you know the horse was once well trained, I'd still be starting at the beginning, ensuring he knows/remembers all the 'basics' before asking/teaching more.
Next, what are you wanting/trying to do with him exactly? Trying to ensure he is decent to be ridden, in order to be rehomed, or...? And what stage are you at exactly? With little detail about what you're asking for, don't know what specifics may be more relevant.
Remember, first & foremost, horses learn by *instant* association, to do what works & quit doing what doesn't work. If they learn something works, they will repeat that. So if he's 'stubborn' about something, that's because someone has taught him - inadvertently or otherwise - that that works. So it will take consistency & repetition, to teach him that it now no longer EVER works. Be utterly consistent with teaching new 'rules', because even if it only works occasionally, that will *strengthen* the behaviour/his knowledge that it works, so teach him to be more 'stubborn'. It's the exact same principle at work that causes people to become 'addicted' to gambling BTW.
So... correct when need be & ensure the behaviour never works, but be patient, considerate & don't blame him or get angry with him - he is doing what he was *trained* by someone to do. Just that you don't desire that behaviour.
Re the bit, firstly I'd be ensuring he went well without a bit. I'd teach him to yield to pressure reliably & softly with a halter, first on the ground, then from his back. Only after he well understood that, would I start teaching him to respond similarly to a bit. Otherwise, you'll end up hurting him with strong bit use, to get the message across. If you haven't taught him well to yield to pressure *gently* from the reins, he has EVERY reason to hate the bit.
Have you first ensured the bit is one that fits his mouth comfortably, then got him desensitised/comfortable to just wearing it, without asking anything of him via it? This needs to happen, or else he has EVERY reason to 'hate the bit', regardless how his teeth are, etc. Of course, possibly he has had good prior education, but it's your timing, lack of release or some such that is causing him to 'hate the bit'.
Hating going away from other horses... Sounds like you shouldn't be trying to do that yet anyway. But you need to first understand his *motivation* for being 'bad'. It is probably to do with fear. Horses are prey/herd animals who don't feel safe away from their 'herd' & out of their 'comfort zone' of environment. If the horse is new at your place, he may not even yet feel fully comfortable/safe AT your place, with your horses. So he's already on edge, so 'pushing' any further will just send him over it. He also doesn't know you at all well, sounds like there has been a lot of confrontation & angst with you already, so you haven't yet established you're a nice, trustworthy person that he knows will look out for him when away from other horses in a strange, scary environment.
So... I'd start out establishing a good relationship and communication with him at home, where he's comfortable & relaxed first, then once that's going well, THEN you can start GRADUALLY teaching him that it is OK, even Good for him, to let you lead him out of his 'comfort zone'.