Brother for Theo...

[rosesarereddish]

In the running is...

Robin
Louis (pronounced Louie not Lewis)
Dominic
Elliot
Isaac
Archie
Elijah

Open to other suggestions!

Robin - this seems so meh to me for a boy. I love it for a girl. And I don't even like unisex names but this one just send to fit a girl better
Louis (pronounced Louie not Lewis) - I love this and think it's great with Theo. I have met a large number of people who do pronounce it Lewis though. They also have no idea at all that it's French 🤨
Dominic - I really love this too and it's so much less popular. I knew a lovely Dominic as a teenager and Dom is such a great nn
Elliot - it's ok.
Isaac - it's ok. I like it but the number of people who misspell it would put me off
Archie - not keen. I still like it the best of the Alfie, Charlie, Freddie, Ronnie genre but it's still a bit dull to me
Elijah - I don't dislike it but I'm not really a fan of very biblical names

I would go for Dominic or Louis.

I love all your names expect Archie. But not a fan of old fashioned names generally.

My order would be:
1 Louis
2 Elijah or Isaac
3 Dominic
4 Robin
5 Elliot.

Other I can think of.
Sebastian
Joshua
William