Hey there! Welcome to the forum.
Its very sweet of you to want to help this little guy out, it sounds like he really needs it. I was actually in an almost identical situation as you are, back a few years ago. The only difference is that my filly was minorly (extremely rough handling) abused as her first experience with humans, which really messed her up.
Is this beautiful little boy a gelding now, or is he still a stallion? That's going to make a big difference.
My advice to you is to a. be persistant and b. to always be a little more stubborn than he is. It sounds to me as if he really has no respect for humans, probably because he's gotten away with pushing people around and scaring them by biting/kicking for so long. You need to win his respect in order to do anything with him.
How? Start by teaching him that he can only do what you want, when you want. This is accomplished with tons of ground work. Make him pay attention to where your personal space it and keep him out of it, teach him to bend away from you when you turn towards him, teach him to yield to pressure, make him back up when you ask, etc. You want him to learn to do something simply because you asked him to, and for no other reason. Make doing the wrong thing way harder for him than doing the right thing.
Once he respects you on a lead line, you ofcourse need to teach him to respect you otherwise, such as when you're brushing him. This was the hardest habit for me to break with my filly, who also tried to kick at me and bite when I brushed her.
I would start out just brushing his neck, or somewhere else that he doesn't mind. Just calmly brush along, and every few strokes go a little bit lower. If he gives you what I call the 'pissy face,' ignore him and continue to brush that area until he once again looks away and looses interest. You might try feeding him hay from a net at first. If he tries to nip at you, immediately make a harsh, dissaproving sound such as 'NAHK!' or 'HA!' and 'bite' him back with either a slap or forcing him to move away from you fast. Next time he goes to bite you, go 'NAHK!' before he even gets in contact with your skin. He'll eventually learn that this is a warning, and he'll loose the urge to bite you because he knows that it hurts him to hurt you.
On to the feet. I started out by touching and 'handling' my mare's feet with a lunge line, rubbing it all over him, from his shoulders to his hooves. Only when he stops fidgiting or giving you the pissy face should you stop messing with his leg. Then praise him.
It takes a lot of work and patience, but there is no reason that he can't be a good boy with time. My mare was known for her murderous disposition when I first started working with her, after never being handled then being messed up by a 'cowboy' who tried to train her by throwing her on the ground. She bucked, kicked, bit, lunged at you with bared teeth, reared, pawed, didn't lead, refused to move, etc. Everyone was to the point of being afraid of her. But I kept working with her and working with her. Now? She's training to be a buggy horse and is one of the favorites out at the barn! She's naturally a very dominant animal, so she does have her moments, but she's hundreds of times better than she used to be!
here is a wonderful article on gaining a horse's respect. It helped me a lot. Four Ways to Teach Your Horse Respect
my mare, now almost 4 years old.
Feel free to message me if you have any questions, I can absolutely try to help you!