My little girls name is going to be Yui. It is so Japanese its not even funny :-D lol so I can relate. Try sitting him down and give him examples of Aiko being used in real life (ex. Aiko Mae 'last name' ) he may come to like it more in application :-)
I like it as her first name. But my husband thinks it is too Japanese.
I like that name :-) it is unique without being cliche. As for first or middle name it really depends on what the name would go with. I would personally use it as a first name, but that is just me :-D