>>At any rate a vet check is definitely in order, along with a neutering. Meanwhile try saying "ouch!" in as high and loud a voice as you can muster when he begins his agression. This imitates the sound kittens make when they are crying "uncle" and have had enough in play-fighting. Most cats will back down when they hear this sound.
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It looks to me like he is seeking attention. I have a cat that does not like me talking on the phone. It may be that they see you talking but that you are not really paying attention to them. She doesn't bite, though, she just meows and gets a little annoying.
At 6 months, he is overdue for a neutering. If you don't do this, he will increasingly become more aggressive. The ouch trick that MA suggested may work too.
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Bob Daisy Emily