I really like the name...

[Rainbowbaby13]

Arthur but it doesn't really go with our sons name which is Dexter.

The middle name will be Nicholas any ideas would be really helpful

Thank you

Look up 'Hurricane' by Bob Dylan - I know I'd heard Arthur & Dexter together...

Dexter and Arthur
Arthur and Dexter

Sounds good to me. Besides they aren't a brand so they don't have to like rhyme or anything

It's fine. Why do the names of siblings have to "go" together? They don't

Well, they do to the extend that you'll be yelling them one after the other across 1001 playgrounds for the next decade at least. Plus writing them on cards etc. It's quite important that they aren't risible together, or actively hostile/imbalanced in some other way.

And if OP thinks they sound wrong together, that's reason enough.

Nico is a lovely name, if it's not a middle name for a particular reason