Aw, thank you for rescuing this cat in need!
And I disagree with Mike - it's not too late now to teach him hands and feet or limbs aren't play things. It will just take time to undo this and retrain him.
The key to his learning manners is
everyone in the house being consistent about the message being sent.
Cats are generally very "me" oriented animals, so an important part of his learning appropriate behavior is going to be providing him an acceptable alternative. It's really like working with young children - they need to be taught not only what is
not OK, but what
IS OK. Being redirected from "wrong" behavior to "right" behavior and focusing on LOTS of positive reinforcement for the appropriate behavior will get you where you want to be a lot faster than any form of "punishment" - which really just does not work with cats. They come to associate you as the problem, not what they're doing.
So with all of that in mind...
A kitty's mom would teach him not to bite or grab with claws by hissing at him threateningly. Humans do not mimic cat hissing well, but we do have a similar weapon, so to speak. It is a short, sharp, puff of air directly in the face. Now, be aware that some kitties lash out at this - but most are startled by it, and pull back. So the #1 "stop this behavior" signal is a short, sharp puff of air directly in the face the moment they begin the problem behavior, accompanied by saying "No" firmly.
It helps to have a lot of cat toys handy - when kittens are teething, we recommend dumping a box of bendy straws around the house so there is always one handy (a good texture for biting). If you have a number of places you can stash wand toys, that's a good idea too, given this is mostly likely play behavior being exhibited. But once you've puffed air in his face and said "no," grab a toy, and redirect him to it. If he goes for it, praise him to high heaven and give him some treats. If he doesn't - try again (unless he's attacking your face - then jump immediately to this next step). If after several tries he doesn't redirect, pick him up, carry him to the bathroom, and put him in there for a five minute time-out. (Don't forget about him). Explain on the way why he's going in there. He shouldn't bite or scratch people! Put him in there, shut the door. When the five minutes is up, open the door without looking at him or talking to him and walk away.
It takes time - but it usually works. Most kitties understand the puff of air directly in the face instinctively... then they get shown what is OK, and get praised for it and get treats for it. If they continue the inappropriate behavior, they get isolated and ignored. Not a big incentive to continue the behavior.
This will not happen overnight as it has gone on so long. When teething at around 4 months, most kittens pick this up in a couple of weeks. I'm thinking a couple of months to really sink in???
All of that said... some people that have had biters have had to resort to a somewhat... unusual method. I've never actually recommended this, and I've never done it, I'm just relating what some people have tried and it seems to have worked. When kitty gave them a good bite - they bit back. Obviously not hard enough to break skin or anything, and everyone seems to do it on the back of the neck. But when kitty bites and you yell OW! and bite him on the back of the neck.... I guess kitty learns that being bitten hurts?
Just another idea to keep in the back of your mind.
Final quick question - does this happen when he's being petted? If so, it is a signal he's being over-stimulated. The correction for this is to stop petting him the minute you see any signal it's too much (often a twitchy tail or something). Sometimes there's no warning. In cases like that, basically you just have to start over. Give one or two pets, pause... and see if he signals he wants more, rather than continue on until he bites/grabs/claws to indicate to stop.
