My Riley is the main chaser here too, if I see him about to pounce while Lu is minding his own business I just call Riley's name to get his attention or make a noise so Lu looks round and is aware that he is about to be hunted!
Do you have a Feliway plug in? Lukey is a funny little thing and has issues about the freezer monster in the kitchen, he started getting a bit twitchy about going in the kitchen recently so I got a plug in again. It has really helped his confidence, as I type he's attacking poor Riley's tail while he's trying to sleep. He has also started initiating the chasing, I guess the difference is that my two have grown up together so the chasing is purely play whereas maybe your Riley is doing it as a territory thing?