How do I revoke his land?

[Vindahl]

I am playing as a tribal chief. My originally high chiefdom was taken from me in a conquest - but when my conquerror/liege died, and his lands divided between his sons, I saw my chance. I raised war on my new liege, claiming the high chiefdom once taken from me. I also won...
Now he then is my vassal, a child chief, sitting in my prison.
I want to get rid of him, take his petty little province for myself. But how to do this... If I banish or kill him, his brother will inherrit, and become independent, as he also is a high chieftain, so this is a no go.
I want to revoke his land, but this is also impossible, as I "have no valid reason".

So what do I do? I want his land, but I cannot get it....

 

[AchedTeacher]

Uh, can't tribals just not revoke in the first place, or am I wrong about that?

 

[A Swedish Goat]

Need higher Tribal Organization, as it works similarly to crown authority.

Though, with it being a child I believe they have to be free to have it removed. Some bug or "working as intended" iirc.