Theo's little brother is called..

[whatshlsnameagain]

Finn (Finley)
Elijah (Eli)
Louis
Rowan
Caleb
Liam

Or something else entirely?

Luke
Jonah

i was going to say Louis before I read your list OP.

Nathan

Keith

Finn

Leo

Hugo

Of your list I like Finn and Eli best.

What about Arlo, Ellis, Wilf or Max too?

Arthur

Another one who instantly thought Felix.

Louis goes best.

Elliot sprang to my mind

I’ll vote Finley as one of the Theo’s I know has a brother called Finley.
the other Theo’s I know have brothers called;
Dylan
Murray
Alexander
Caleb
Jacob
Joshua

Evan?

Elijah

Or
Lawrence (Laurie)
Nathaniel (Nate)
Sebastian

Alec
Otis
Henry
Kit
Duncan

Milo

Ralph

Theo is very popular so something similar would be

Harry
Finn
Jack

I'm sorry but I don't like any of your choices
What about Alexander? Max? Sebastian?

Louis or Seth

I think Finn is lovely on a boy

Theo and Max (Maxwell)
Theo and Pip (Philip)

@whatshlsnameagain I like all the ones you suggested. Here are some more I like for Theo:

Tiernan
Conan
Dara
Lorcan
Callum

Theo & Finley from your list

or

Theo & Zachary
Theo & Archie
Theo & Alfred (Alfie/Freddie)
Theo & Dylan