Yes, I think HE is afraid...

remember it is two against one... and he may have forgotten how to "play"...and needs to get back in that groove. Kittens are unpredictable, and this may
upset him too.

You might try putting one kitten in another room and introducing the other one to your established one. One on one may be easier for him.

I had a vet once tell me, that cats do this, and even play rough..what we think is aggression is really just their way. Sometimes the screaming etc is fun for them. Our middle kitty used to scream and wrestle with Roofie (a cat we no longer have--killed
.
by a police car who came up off the road under the bushes to hid and radar cars).. they had grand times. She was so depressed after his passing, we got another Oreo...and when Oreo played rough, Tippy did her scream thing, and scared Oreo right off.. Oreo won't play that way anymore. But they do hide and seek very well, instead! So some learning and personality differences do exist. Our oldest cat has pain issues, and will hiss off any play advances..she is 16. So do make sure your established cat is not ill or hurting. That may affect the introductions alot.

We have cat trees so any of the cats can go up high when they want privacy. You might consider building one, or buying one. They work well.