#Single point of failure
Single point of failure
The big issue I see with using an instrument for verbal components is that it effectively replaces both the material and verbal components with one item. and as the hands that hold it can also do the somatic components this puts all parts of the spell in one place.
This situation is actually made worse by also allowing him to make it his weapon because now he gets a bonus in the item economy. Instead of having to work towards getting better weapons and a better focus possibly having to attune to both he now gets both in one package.
##Alternative: horn
Alternative: horn
The second issue can somewhat be mitigated by using a one-handed instrument like a horn (Viking, not French), as this still requires him to use his mouth and have a hand free for a weapon.
##Alternative: Whistling
Alternative: Whistling
My personal favorite would actually be to allow him to still effectively use his mouth for the verbal components but instead of words him whistling. If he uses it to cast mage hand or something he can do his best Yondu impression.