Hi Bex
Sorry to hear you're having BT troubles.
Maybe it could have started being a touch UT but yeah, sounds like poss OT now. Both mine had big drop in daytime sleep needs (to preserve reasonable BT) at 18 mos. Well, dd2 is there now really. Too much nap stole more from their nights so they got less sleep overall.
Dyt anything else could be going on that could be making settling harder? Eg language development, teething?
How does he settle for naps?
Does he have a lovey? Could that help maybe? Does he settle independently? How have you been handling it when he has trouble? Can you do WIWO or do you need to stay with him?