I think [name_m]August[/name_m], [name_m]Hugo[/name_m] and [name_m]Rupert[/name_m] would be perfect as a little brother to [name_m]Finley[/name_m].
[name_f][/name_f]I’m not fond on siblings sharing initials so [name_m]Fraser[/name_m] is the least appealing to me on your list (additionally it is definitely a different vibe to [name_m]Finley[/name_m] imo)