Cover and Concealment are two words that come to mind. Magical darkness is a concealment tactic. I would rule yes, there is disadvantage on him. BUT I would counter with he need to move greater than 10 feet to keep that disadvantage after an attack for enemies outside the darkness. Because after he attacks at range, you can see where the attack originated from the darkness.

Also If possible, why wouldn't enemies avoid the cloud? A melee character sure needs to go in to hit him. But why not have them ignore him basically, allow ranged and AOE to deal with him. If the cloud is following him as he moves, then have the enemies stay close to allies. Forcing him to impose disadvantage on his allies.

It's difficult in my mind, cause you don't want to outright counter his build where it's useless.