We haven't hit this stage yet (we just transitioned to toddler bed 2 months ago), but it is my fear!!
If (when!) it happens with my son, we'll do a reward chart to motivate him to stay in bed. Your little guy is pretty young (20 months if my math is right?), but it may work with him too. Stickers for every sleep period he stays in his bed until you come tell him it's time to get up. 3 stickers = prize (matchbox cars work for my son, but you could do a gummy bear, trip to the park, or small toy/stuffed animal you know he'd like).
If you do 3 stickers = prize then he has a chance to get a prize pretty much every day. Once you have a good week of correct behavior then you can do 5 stickers=prize and only offer sticker if he remembers. That way he'll slowly forget about the reward system and you'll just have him doing as you ask!
Otherwise you may have to spend a little more time in his room to ensure he's sleepy.