@girlmom21
Their names don't need to 'go'. They're individuals.
I think they're fine together anyway.
What about Archie?
Perfectly put by girlmom21.
I love the names Dexter and Arthur. If you do, then go for it, but obviously no harm in exploring other name suggestions. If he isn't here yet, then you may change your mind when you meet him anyway!
I can think of some people I know that have multiple children, I don't wish to list their names, but their first child's name is very traditional, their second child more modern and their third I had never heard before, so I wouldn't have said they particular 'go' together, but I had never looked at it that way before, until trying to think of children's names right now.
Hugo, Logan, Hunter, Harrison, Carter and Cody are what I would say were more modern names that aren't hugely popular but also lovely.
Whatever name you choose, I'm sure it will be perfect for them.