Recently neutered 5-mo. old kitten mounting spayed 5-mo. old female kitten?

I have two 5 month old kittens from different litters (both were rescue cats); the female was spayed long ago, but I only recently neutered the male about three weeks ago. He had always jumped on her back before he was neutered, but he never tried to , he looked more like he was trying to get a piggyback ride. Last night, he mounted her on my bed and did . Why? And, more importantly, will he grow out of this?