Male cat attacking others

We have a 2-year-old male cat, Sam, who is neutered. He is very sweet around us, but lately he has started to pickon our two female cats (also fixed) named Mia and Isabel. He gets on them their backs in the mating position and grabs on to their necks. He also bites at them and picks on them. He has never done this before the last 2 months or so and I can't think of anything that has changed. We've had Mia since we got Sam (they were from the same house, but not littermates) and we've had Isabel for about a year and a half. It has gotten bad enough that we often have to put him in their room and shut the door. He hates to be alone and so he cries, but we dont' know what else to do.