Here's the real difference...
Bucklers and small shields have all the same bonuses and penalties, by the numbers. AC bonus, Armor Check Penalty, Arcane Spell Failure... they are all the same.
Bucklers, however, will let you do something with that hand with out having to drop the shield... You can fire a bow, swing a weapon two-handed, wield two weapons, drink a potion, etc.
Meanwhile, with a small shield, you can carry something in that hand (a torch, for example) but not wield a weapon with it. Also it can be used to make shield bash attacks as an off-hand weapon, and it can have shield spikes attached to it... Bucklers cannot.
You should not allow him to dual-wield when wearing a small shield (even a mithril one), or use a buckler to shield bash. It's one or the other.