Arthur's brother should be called....

[ChittyBB]

Wilfred (nn Fred)
Hector
Max
Fergus
Thomas
Samuel (nn Sam)
Jack (family name)
Lawrence
Humphrey (I love this but don't really have the guts to use it)
Byron (family name but I'm just not sure that it isn't cheesy)

Ford.

Merlin

From your list, Wilfred!

I do like Humphrey too!

From your list, Lawrence. I'd spell it Laurence though.

Lancelot

Laurence or Stanley

From your list Sam and I think Toby would be nice too.

I really don't like old man names at all I think they sound ugly. Why anyone would call someone Fred is beyond me

Gavin (the modern version of Gawain)

Arthur is named after my Grandfather and Great Grandfather who was an amazing man who deserved remembering.

I have it when people assume we did it due to Merlin obsessions but I suppose that was inevitable. I'm certainly not compounding that connection with another name!

Love Your list. Also love

Bertie
Hammish

Not keen on Sam or jack but like all the others. Love Humphrey and would use wild as nick name for Wilfred.

What about:
Hal
Rufus
Llewelyn
Clive
Morris
Digby
Harvey

In an ideal world I would like to keep a family connection. I like names that connect you to ancestors. But that would mean using Jack, Byron, John or Kenneth and I don't think any of those are quite right.

I think Fergus or Jack.

Benjamin

I love Hal. But what would be the full name?

Bertie is sweet too but again, what would be the full name? Egbert is awful and Bertram is weird.

I thought Max before I opened the thread to see you list

Thomas

Lawrence

Hal is short for Henry I think.
I love it but not keen on Henry so would have gone for just Hal but dh vetoed it.

Thomas or Wilfred

I like Wilfred. Also Hugo, Isaac, Theo

Kenneth is nice and quite unusual these days. Kennie for short.

Albert. Bertie.

If you have Arthur and Henry. They can be Art and Hal for short. Both cool names.

You could have Bertie as shout for I knew an Edward who when Bertie Robert, Albert, Herbert oh and herby is cute too...