5 month neutured (and shy) kitten mounting spayed and outgoing sister?

My 5-month old male kitten started biting his sister's neck and laying on her (I interupted him before he started doing anything else - if he had intended to). This happened after she was running around a lot during play, so was excited and panting. He tried to do this twice. He does not play with the same intensity and is fairly lazy, so was not in the same state.

I have read that this is how cats can show dominance, but what I find weird is that she seems to be the more dominant of the two. She is much more social, outspoken and a "leader". She is fairly fearless while he is timid and hides. He often mimics what she does rather than the other way around. They usually get on very well and play a lot.

So, does anyone think if this could be something other than dominancy? Or is he really an alpha male in scaredy cat clothing?  

They were both fixed about 5 weeks ago.

Something that also might be worth mentioning is that the boy also is a slower developer and looks much more like a kitten than his sister who looks and acts more like an adolescent.

Thanks so much for any insight.

Hormones often hang around for 6 to 8 weeks after neutering in a male.

I think he'll settle down.. he's just sowing the last of his wild oats..

Yes, I caught him doing it again tonight and he was aroused. It seems to happen when she is excited and playing. She must be emitting a pheromone that attracts him. I heard of applying Boar Mate which makes a female cat smell like a male one and they are usually left alone from sexual advances after that. I don't know where to get it by me, but it could be a good idea. I really do not like to see him try to have sex with his flesh and blood sister!