I would do lots of large rectangles using the whole arena. Ask for straight fast gallop down the long sides and collect him for the ends. This should stop him anticipating the stop. Also work on speed control on the large rec too. Five fast strides, 4 slow, back to fast. This will keep him thinking and listening to you as right now he is associating a fast run down the arena with a stop. Also try fencing him, run ALL the way to the fence before you say whoa. I also do lots of trot stops, back up, roll 90 degrees and repeat. This really helps them wait and listen and gets their hocks under them.
Be sure when you do run down to a stop again after doing lots of the above exercises that you are building gradually from the turn and not exploding half way down and letting the speed fall before the stop. Check you are not stopping riding in your run down too, keep driving all the way to the whoa. On video I noticed I was anticipating the stop myself so would still my seat just before the stop causing my horse to slow, which makes for a uncommitted stop.
Carrying The Blood of the Steeldust Line...I Know My Silent Partner Stands Behind Me All the Way...I. Tyson