I'd really cut the chat in the middle of the time. Basically do silent return, so the first couple of times you might say, "Moon is on the clock, it's sleep time". After that just say nothing and return him to his bed. Overand over and over again.
WRT rewards, someone told me that with sleep in particular, rewards work best when they are immediate. So for example the 'Sleep Elves' have told mummy that they will bring a little (sweetie of some kind) to little boys who stay in their bed until morning. So the reward comes immediately once it's morning, rather than having to wait a week.