Quartz
Hero
My battlemaster player has been saying that he doesn't feel like he has enough control in combat with only one reaction.
Fighters - particularly Battlemasters - do need help in 5E.
Give him an item that lets him take a second reaction and see how it goes. If it's too powerful you can remove the item; if it's good then you can make it a class ability. I'm not so hot on making it a feat as I can see the opportunities it would give a Rogue. In an earlier thread I riffed on Fragarach with an item that gave the wielder extra Riposte strikes as sort-of-Legendary Actions.
Battlemasters get very few maneuvers and superiority dice, so an alternative might be to increas the numbers of those. Balance this by dropping the feat / ASI at 6th level.