William is lovely - very classic with some great namesakes. It's always stayed in the top 100 so is very timeless. Arthur is nice and also classic but much more subject to the whims of fashion - it's currently at an all time high so might date your DS more.
Kit is lovely too but so trendy at the moment as a given name - the older Christopher, nn Kit might serve him better longer term imo. Otto is too German comedic sterotype for me personally, sorry.
Similar to William - Edward, Alexander, James, Thomas, George, Charles (all classics that have stayed in the E & W top 100 since records began).
Similar to Arthur - Frederick, Henry, John, Robert, Albert, Peter, Phillip, Alfred, Wilfred, Stanley, Sidney.
Similar to Kit - Bertie (Robert/Albert), Pip (Phillip), Jem (James/Jeremy), Hal (Henry), Cal (Charles/Calvin), Ned (Edward or other Ed- name/Benedict).
Similar to Otto - Ivo, Jago, Hugo, Nico (Nicholas/Dominic).