I think all your names go nicely with your girls names. So, just to be picky, I'd throw out Charlie bc with Finley as a DD name, you might think girl for Charlie as well. I'd throw out Felix for the matching initial with Finley. Leaving Carter, Sawyer and Penn. And since Finley is a more unusual choice for a girl, I'd go with Penn as a more unusual boy choice :)
With the spunky duo of Finley and Eliza, my first choice would be Sawyer. I don't know if two occupational names is ideal though (Sawyer Mason). If you don't mind it, that would be my pick! I love Felix but it shares an initial with Finley. Charlie Mason sounds like Charles Manson! Yikes.I'm not a fan of Carter or Penn as first names.
I agree with Flaunt. I think Penn Mason sounds awesome! Unique like your girl's names, but clearly masculine. My initial reaction to Sawyer was positive, but Sawyer Mason sounds a little...hmmmm, labor union-ish? :) Only because I believe the names mean "woodcutter" and "stone worker" respectively. He'd be a hard worker, for sure!
If you are set on Mason as the middle, I have to say that none of the names on your list sound right with it.
If I didn't know this or your other children's names, my pick would be Felix.
If you are open to suggestions, here are some names that you might like:
Max (Maxwell, Maksim)
Ben (Benjamin, Bennett)