You can come back with Divine Abduction and "Spirit him away" with it, but I'm not sure where he goes or if you ever see him again.

There are two pin wheels you can find in this area, a Red and White one and a pure white one. Given the red and white pin wheel he will state it is the wrong one and tell you more about the white pinwheel. He then has two options, He then can be given the White pinwheel or a new option to send him to the dungeon appears. If you give him the white pinwheel he thanks you and asks for you to kill him. Only option is to refuse. There is no option to send him to the dungeon after this altering his quest line. He can be Spirited away with the Divine Abduction prosthetic.

Alright I have done another run through and there are more options to send Kotaro too. This one is a little long. Anayama The Merchent you meet in Ashina outskirts before the first Red eyed Ogre. Talk to him and purchase his Info, Rest and he will have items for sell. Purchase those and he states if he can find a way to make more money he can bring in new products. The next part is going to Ashina Castle towards the back of the castle before you get to the serpent shrine there are two guards on a bridge, use eavesdrop and the guards talk about needing more salt. Relay this info to Anayama, rest and he will have new items for sell. Once again buy his items and talk to him. He will state looting corpses can be a lucrative business but needs someone big, strong, and well dumb. Now following the below path of providing kotaro the wrong pinwheel you can send him to meet with Anayama the Peddler.

After you spirit him away, he is with the Folding Screen Monkeys, and he thanks you with some item i forget right now

What does the "Taro Perssimon" do? The page for it doesn't exist, no one is talking about it online, and I want to know which pinwheel I'd rather give him

So I have both pin wheels n idk if i broke it but i cant give him***** when i try n talk to him he just complains about ppl leaving please help

how do i spirit him away? is this something i havent gotten yet?

IF YOU WANT HIM TO WORK DO NOT GIVE HIM THE WHITE PINWHEEL I *****ed my quest because I thought I had to give him both pinwheels and I gave him the white one first.

I spirited him away after giving him the red flower, now in the dreamworld with the monkeys he just sits there and says "Ahh..." i now have the white flower but cant interact with him, did i mess up? :c