Huh. I've not had an issue with Faendal equipping the upgraded bows and arrows I've given him, though he will randomly decide "EFF THIS NOISE" and rush into battle no matter how far out of range he was with his bare fists.
I don't really do followers anymore, they're nothing but a nuisance outside of the games specifically designed for large party combat. Either they're clipping me with their attacks, ala Fallout 1 & 2 or they're clearing out entire rooms before I've even gotten off the load screen (Damn you, Boone.) I was kind of surprised when I brought Faendal along on my last character so we could be Bosmer Bow Bros how dangerous he actually is, I ended up falling way behind on my own Archery because he'd shoot everything down.