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I'd go with a name with more than 2 syllables and something that doesn't end with a similar sound (Kieran is a FAB name, BTW)
Kieran Alexander
Kieran Anthony
Kieran Amadeo
I know you asked for A names, but I kind of love Kieran Jameson.
Re: A-names
Ashton
Anthony/Antonio
Adair
Alfred
Aidan (too common, but fits your criteria)
Andrew/ Andre if you want to be creative
I think Aspen is just fine as a middle name. If that's your style, use it! How often is he going to be introducing himself with his full name anyway?
Kieran Absalom
Kieran Adir
Kieran Ahab
Kieran Alaric
Kieran Alec
Kieran Alistair
Kieran Aric
Kieran Anthony Durgin *
Kieran Augustus Durgin *
Kieran Albert Durgin
Kieran Adam Durgin
Kieran Abram Durgin *
Kieran Artie Durgin
Kieran Arthur Durgin
Kieran Alexander Durgin *
Kieran Ames Durgin *
Kieran Apollo Durgin
Kieran Archer Durgin
Kieran Asher Durgin *
Kieran Atticus Durgin *
*my favourites
I'd go with a name with more than 2 syllables and something that doesn't end with a similar sound (Kieran is a FAB name, BTW)
Kieran Alexander
Kieran Anthony
Kieran Amadeo
I know you asked for A names, but I kind of love Kieran Jameson.