Alright, so Percy is a son of Poseidon (obviously), and can control water (again, obviously). It was shown he can control poison in Heroes of Olympus, and he can also control rivers in the Underworld (albeit with some difficulty). The rivers in the Underworld should have barely any water in them, but they might even have none.
Since he controlled the Lethe, that means he should be able to control liquids, minus those that do not contain water (gasoline, mercury, bromine, etc, molten metals, etc). There are a lot of liquids. Here’s a nice list: https://thingdb.io/things-that-are-liquid/
Since the human body usually contains around 55 (adult females) to 60 (adult males) percent water, Percy could theoretically choke somebody in their own body. Blood is 90% water, which means Percy could probably control it. Percy might be able to blood bend. I find that really freaky.
Percy should also be able to control ice (the solid form of water, I believe this actually happened in SoN), and water vapor (the gas form of water).
I wonder if Percy could control plants, as they contain 90% of water. That might make him too powerful.
Of course, this all depends on if Uncle Rick does his science research or not. Or if Uncle Rick decides he’s made Percy too powerful (unlikely).
Anyways, I hope you enjoyed this!
TL;DR: Percy should be able to control almost all liquids, because almost al of them contain water.