Sixth child doesn’t help you much unless you want to go with [name_m]Sextus[/name_m] 
[name_f][/name_f] or Sixtus…you could also go with a name that has six letters!
Second son meanings…
[name_f][/name_f][name_m]Thomas[/name_m] (‘twin’)
[name_f][/name_f][name_m]Deuce[/name_m]
[name_f][/name_f][name_m]Twain[/name_m]
[name_f][/name_f][name_m]Jiro[/name_m]
[name_f][/name_f][name_m]Kenji[/name_m]
[name_f][/name_f][name_m]Malachy[/name_m]
[name_f][/name_f][name_m]Secundus[/name_m] / [name_m]Segundo[/name_m]
‘Eight’ meanings…
[name_f][/name_f][name_m]Octavian[/name_m] / [name_m]Octavius[/name_m] / [name_m]Octave[/name_m] / [name_m]Ottavio[/name_m] / [name_m]Otto[/name_m]
[name_f][/name_f][name_m]Hachiro[/name_m]
[name_f]December[/name_f] associations…
[name_f][/name_f][name_f]Winter[/name_f]
[name_f][/name_f][name_f]Eirian[/name_f] (‘snow’)
[name_f][/name_f][name_m]Robin[/name_m]
[name_f][/name_f][name_m]Tollver[/name_m] (‘twelve’)
[name_f][/name_f][name_m]Hollis[/name_m] (‘holly bush’)
[name_f][/name_f][name_m]Cymbeline[/name_m] (‘sun lord’ for the winter solstice, Cym is a cool nn)
[name_f][/name_f][name_m]Heliodor[/name_m] (‘gift of the sun’)
He’ll be a Sagittarius, so you could also use something like [name_m]Archer[/name_m], [name_m]Bow[/name_m], [name_m]Fletcher[/name_m], [name_m]Evander[/name_m], [name_m]Ivo[/name_m], [name_m]Arrow[/name_m], [name_m]Ignatius[/name_m], [name_m]Aiden[/name_m] (‘fiery’!), [name_m]Blaze[/name_m]
[name_f][/name_f][name_f]Hope[/name_f] that helps!