I've done a bit of archery in my time and having a lump of wood on either arm would be pretty cumbersome to firing a bow (or even a cross bow). Also trying blocking anything with a buckler strapped to your arm, it makes no sense without a hand to grip it in place!
Even if the player argues its strapped to their arm, you should still impose some form of additional difficulty to all ranged attacks (maybe a black dice or two).
The rules say bows and cross bows are two handed weapons, I have not looked up bucklers, but I doubt they are no hand weapons.
The argument your player is making is comparable to someone waying "I'd like to use my 2 handed great axe and and then block with the towershield strapped to my back". Its a little bit cheesey.
I'd say refuse the buckler, give him large ranged penalties (and only allow him to parry), or make him use a maneuver to swap weapons when he goes into combat (but don't let him block while using a bow!).
EDIT: Most systems i've seen using bucklers give them less stats for defence but the bonus of being able to parry with them (rather than using a sword). I don't think Bucklers should be used to block anyway.. they are tiny!