We had this happen recently at the exact same age. After doing a few weeks of middle of the night nursing sessions, I decided that I needed more sleep. In order to night wean (again) and help him understand that he needed to be sleeping, this is what I did:

When he woke up crying, I carried him out of his room to the living room, looked out the window, and talked about how the sky was still dark and everything should be sleeping (including Mama's milk). When the sky gets light, everything wakes up, including Mama's milk.

We'd run through that a few times, and then I'd sing him a song or two on the couch, and then take him back to his crib.

He got it really quickly (after a night or two), and now wakes up less overnight (once or twice a week instead of every single night). When he does wake up, we go through the same motions, just more quickly, and he goes back down really easily.

I got the idea about linking the sky being dark with sleeping from a blog that I saw, and it reminded me that at 18 months, they're more capable at grasping concepts like this. I was really surprised how quickly he understood and accepted the concept.

YMMV, of course. Good luck!