Sammy, age 18 months, was neutered at 6 months. In the last few weeks, he has started mounting his 2-year-old cat "sister" and 11-month-old cat "brother." They're both a lot bigger than he is (by 2 to 5 pounds) and get angry when he does this. It usually leads to a lot of snarling, flipping and chasing, often late at night. Before this, they all got along pretty well. He used to curl up with our female cat to nap, but now she doesn't trust him. I worry that this behavior could lead to an honest-to-goodness fight, and he is so small, he could get hurt. How can we discourage this behavior, since he doesn't seem to get the message from his fellow cats?