Hi there, some do drop some sleep around 18 mths, it can be a wonky time. What's happening with his teeth? If he's not teething then perhaps the EW is due to his nap being too long.
I would try with cutting back the nap a tad, maybe 2 hrs? You will need to figure out where the nap needs to fall in that case.
In the meantime, I would bring bedtime back a tad or he's just going to get really OT.