Your cat is biting you when you play with him? At first its a bit of a giggle, but then he crossed the line and you happened to forget that your cute little kitty is in actual fact 4/5 wild animal. Take away the collar and let him go without food for a couple of days and see how cute he is then.
Why is your cat biting you? Could be a number of reasons.
One in particular, may well be that you are using your fingers as toys, it doesn't take much to activate his hunter instincts, just try dragging a piece of string in front of him for a few seconds and watch his stance change and his pupils dilate to the size of quarters. Your cat is still fairly wild and always will be, he just puts on his civilized demeanor when he is in the house. Try using a toy with a decent reach or some string or something that will keep your fingers away from his mouth.
Another reason he could be biting you is actually out of affection. Have you ever seen footage of tigers or lions playing? If a human happened to be one of them, he would not last five seconds, yet they are still just playing. Cats, both big and small, are tough as old boots and what seems like a bit of fun or affection to them will more than likely be blood loss to you. If he seems to be getting a little too wild, don't scorn him and send out the wrong message, just take it down a few notches
Another classic reason why your cat bites, is that it is basically annoyed or p-ssed off. Look for its body language, is its tail thrashing? Is its fur on its back up a little? Is it moving away and banging up against the nearest solid object? All these are indicative of displacement gestures and are sending the message that it is getting angry and that it could well be time to stop playing for the time being.