I don't like any of them, to be honest (you're right about the 'grandpa' factor), but any would fit with the others, and I think
Baxter fits okay. Not as seamlessly as
Barnaby or
Bernard, perhaps, but just fine in my opinion, especially with
Daphne.
Other names to consider:
Benjamin (popular, yes, but not as much as it used to be)
BennettBenson/Benton
Benedict/Benedikt
(these ones are nice because he can use
Ben, but it's still in line w/ the sibs styles)
Brenton (ditto above, but
Brent as nn, or Bren)
Bryant (ditto, but with
Bryan)
BarrettBeckettBranson (not as typical and modern-sounding as
Brandon, but still able to use nickname
Bran - game of thrones popularity won't hurt - or if no nickname needed, it's not too long)
Bridger (really different, but could be very cool)
Bryce/Brice
Bastian/Sebastian
Gerard
good luck!