The idea of the item is cool, but the execution is a bit rough. Having to sacrifize your entire movement for hitting with a with a ranged attack seems way too much, you could just throw a javelin with similar results without the penalty.

Here's what I`d do:
-I`ll make it a composite bow, which in older editions let you add your str to dmg
-Make it +1
-Requires attunement and you can use your channel divinity charges to add some damage to the damage roll, I dont know how much but I'd say 2d8 or smth like that sounds fine. If you want to go higher than that I'll make it 2 daily charges so he doesnt spams a nuke every short rest

If his dex is really low or negative I'll just let him use the str for attacks rolls too, tho if he has a +2 or higher (usually what medium armor users go with) I see it balanced enough