Sasha Leonardo "
Leo" is a really good idea, I like the sound of it.
Keeping
Arthur as a fn is also a good idea, because it's already his name afterall (with an extra h).
Arthur Oberon and
Arthur Casimir would both be great :)
Some other ideas for
Arthur (And I'm guessing 'or something similar' means something about the same length):
Arthur Erasmus (means 'beloved' :)
Arthur ValerianArthur GovannonArthur SalvadorArthur SpiridonArthur CaspianArthur EleazarArthur Rafael (maybe too Ninja Turtles with
Leonardo?)
Arthur MordecaiArthur ChristianArthur JaromirArthur OctavianArthur MalachyArthur ConstantineArthur MarloweArthur LucianArthur Firenze (It's listed as a girls name, but I prefer it for a boy)
Arthur ConradArthur Gabriel