First of all, thank you for rescuing this kitty!
Secondly - if you have not seen a Doc about the bite, PLEASE keep it clean, put neosporin or some type of antibiotic on it, and keep a VERY close eye on it. Cat bites can be very serious, and if you have ANY doubts about how it's healing, get to a Doctor. Tell them you were trying to pet a stray cat somewhere, not that it was your pet. I don't know where you live, but in some places if your cat bites you it must be quarantined at a vet for some period of time.
There are any number of things that could cause this behavior.
When feral, cat moms will begin to push their children away at around 4 months. He does not see you has his mom cat, but you are is caretaker... this could be a territory issue. Having him neutered sooner rather than later may go a long way to resolving this problem.
Could this be redirected aggression? Is there a stray or feral cat outside he can see or smell (or both) that triggers this?
Does he get a lot of stimulation? This could be his "hunt and prey" instinct kicking in... do you give him "play time" every day? Assuming he's an indoor-only cat, playing with him for 15 - 20 minutes every morning and evening with wand toys or whatever stimulates him and gets him moving may also help stop this problem.
If it's not any of these things....
Have you changed anything about your scent recently? Do you smell like another cat or animal for any reason? You're his caretaker and provide his food... has anything
changed? How long has this been going on? Are you a woman - and does this coincide with "that" time of the month? Has anything in your home changed? One person found that the change that caused the aggressive behavior in their cat was bringing a bicycle inside. Who knows what smells were on the tires... but bringing in a bike is not something most people would think could cause sudden aggressive behavior in their cat. So think hard about any changes on you, with you, about you, or in the environment - which he thinks of as his territory.
Also, at 5 months he may be teething. That's usually around 4 months - but not unusual at 5. This is NOT normal behavior for teething... but you never know, I guess.
It could also be a health problem on his part. Some cats get aggressive - especially if they're in pain for any reason. I realize he's only attacking you - but as his caretaker, it would be your attention he needs to get. Maybe move up that appointment for neutering - and get him a full check-up to go with it?
No matter what the cause - it needs to be managed. Yelling at or "punishing" him will not work - or even help, for that matter. Yelling "OW" is of course OK. He needs to know he's hurting you. For your future defense, you need to know that the fastest way to get a cat's mouth off of you is not to pull, but to push. Pushing in forces the jaw open and he should release you.
The normal advice for biting or teething cats is to blow a short, sharp puff of air directly in their face, and then redirect them to something that is OK to bite or chew. A great "redirection device" is bendy straws. Buy a box of them, scatter them around the house, and keep 'em handy.
With cats, "punishment" per se does not work. Cats are not pack animals, and therefore, unlike dogs, they are not "preprogrammed" to want to make us happy. They are about territory, and they are about "me." They can love people - and some kitties do want to make us happy. But for the most part, to "teach" them anything, you have to work on the basis of redirecting them from bad behavior (biting a person) to good (biting a straw) or denying them something they want (attention). If kitty doesn't redirect - he gets a time out in the bathroom for 3 - 5 minutes (because of the more serious nature of the problem here, I'd consider 5 or 10 minutes to let him calm down if this is just "hunt prey" instinct issues) - or you just walk away and ignore him.
But they also learn really well from positive reinforcement - so praising him for all the good stuff he does is really important. Helping use up that energy in constructive ways is important - and praising him a lot for and during proper play is important.
Here's a thread where someone else was experiencing random attacks from their cat - you may find it helpful:http://www.thecatsite.com/forums/sho...d.php?t=152718
...but I would get him neutered with a full vet check-up - and as soon as possible.