R109: The section labeled "Improvised Weapons" (PHB p.148) states that, when a melee weapon without the Thrown property is used to make a ranged weapon attack, its damage die becomes a d4. That might not mean that it counts as an improvised weapon, but if it does, the attacker would only add his proficiency bonus to the attack roll if he were proficient with improvised weapons. Either way, he would add his Dexterity modifier, not his Strength, to the attack and damage rolls.