[name_m]Hi[/name_m] there.
I know how you feel about so many little boys. All on dh’s side, we have 13 nephews & one niece. Tons of great boy names taken!
Here are my thoughts-
[name_u]Miles[/name_u] [name_m]Christopher[/name_m]-This is a nice name & goes really well w/Kellan. I do think that it’s pretty popular though. I feel like I meet little tykes called this a lot.
[name_m]Daniel[/name_m] [name_m]Paul[/name_m]-Also kind of popular. [name_m]Daniel[/name_m] is a beautiful name, but I’m w/you. [name_m]Dan[/name_m] & [name_u]Danny[/name_u] aren’t nearly as good. They may be somewhat unavoidable. If you preferred [name_m]Daniel[/name_m] but were okay w/the nns, I’d say go for it. But if you really dislike them I wouldn’t use [name_m]Daniel[/name_m].
A few other ideas-
[name_u]Alden[/name_u]
[name_m]Archer[/name_m]
[name_u]August[/name_u]
[name_m]Adrian[/name_m]
[name_m]Alistair[/name_m]
[name_u]Brennan[/name_u]-too rhyme-y?
[name_m]Brendan[/name_m]-too rhyme-y?
[name_u]Casey[/name_u]-love this for you!
[name_m]Dawson[/name_m]
[name_m]Declan[/name_m]-sort of popular, but a great name & super cute w/Kellan.
[name_u]Dexter[/name_u]
[name_u]Emmett[/name_u]
[name_u]Finlay[/name_u]-getting a bit common for girls, so that’s a bummer. I love it for you though.
[name_u]Flynn[/name_u]
[name_m]Ferris[/name_m]
[name_u]Grady[/name_u]
[name_m]Gibson[/name_m]-I love this one. I rarely suggest it for fear of over exposure, but I made an exception. 
[name_m]Gareth[/name_m]
[name_m]Holden[/name_m]
[name_m]Harrison[/name_m]-I mainly love this for the nn [name_m]Hank[/name_m].
[name_u]Jameson[/name_u]/Jem
[name_m]Lawson[/name_m]
[name_m]Lachlan[/name_m]
[name_m]Malachy[/name_m]-one of my many Irish named nephews.
[name_m]Magnus[/name_m]
[name_m]Nolan[/name_m]-one of my all time favorites. Maybe too close to [name_m]Kellan[/name_m]? Or maybe the perfect fit, I’m not sure.
[name_m]Nathan[/name_m] nn [name_m]Nate[/name_m]
[name_m]Nathaniel[/name_m] nn [name_m]Nate[/name_m]-I mainly think that [name_m]Kellan[/name_m] & [name_m]Nate[/name_m] would be cute, but it needs a full name.
[name_m]Patrick[/name_m]
[name_u]Quinn[/name_u]
[name_m]Rafferty[/name_m]
[name_u]Rafe[/name_u]…or [name_m]Rafferty[/name_m] nn [name_u]Rafe[/name_u]
[name_u]Sullivan[/name_u]/Sully
[name_m]Seamus[/name_m]-another nephew!
[name_u]Tristan[/name_u]
[name_m]Thatcher[/name_m]
[name_f]Hope[/name_f] this helps. 