Both options are handsome! They have very different vibes to me – [name_m]Callahan[/name_m] is very down to earth, relaxed, and friendly (love the nickname [name_m]Cal[/name_m] as well!), while Zimmerman feels debonair & vintage with a bit of spunk, though he is definitely a little heavier to wear.
[name_f][/name_f]In all honestly, I do think [name_m]Callahan[/name_m] [name_m]Jack[/name_m] is my preference, especially since he aligns a lot better with [name_m]Roman[/name_m] & [name_f]Briar[/name_f] vibe & popularity wise.
A few middle name suggestions anyway… I think you’d need something a little lighter & playful to balance out the heaviness of Zimmerman.
[name_f][/name_f]Zimmerman [name_m]Louie[/name_m]
[name_f][/name_f]Zimmerman [name_m]Jack[/name_m]
[name_f][/name_f]Zimmerman [name_m]Jude[/name_m]
[name_f][/name_f]Zimmerman [name_m]Alec[/name_m]
[name_f][/name_f]Zimmerman [name_m]Asa[/name_m]
[name_f][/name_f]Zimmerman [name_m]Eli[/name_m]
[name_f][/name_f]Zimmerman [name_m]Heath[/name_m]
[name_f][/name_f]Zimmerman [name_m]Levi[/name_m]
[name_f][/name_f]Zimmerman [name_m]Hugo[/name_m]
[name_f][/name_f]Zimmerman [name_m]Luke[/name_m] / [name_m]Luca[/name_m]
[name_f][/name_f]Zimmerman [name_m]Theo[/name_m]
[name_f][/name_f]Zimmerman [name_m]Charlie[/name_m]
Best of luck, hope this helped!