Introducing New Kittens to Resident Cat?

In April I adopted my 6 month kitten Raven and I've had him since he was 8 week old. He's not really been around other animals, unless we've take my in-laws' dog in while they've gone on holiday.

Last Thursday I adopted two new kittens about 8 weeks old, a brother and sister who play really nicely with each other. The idea was that Raven would become more sociable and have some friends, but now he keeps attacking them both by grabbing them, chasing them, batting them, and biting them to the point they squeal.

At night we let Raven sleep with us as he's always done since we got him. We shut the door for the living room and leave the two younger kittens in there alone and we do the same while we're out. When we're home we let Raven into the living room with the kittens, but within 5 minutes he's attacking them or stalking them.

We tell Raven "no", but he won't stop unless we come over and get him off the kittens, I'm scared that he's never going to accept them or hurt them.