My horse likes to race at jumps and after we land he gets unbalanced and when I ask him to slow down he usually just talks smaller but still fast, strides. (Ridden in a gag)

Like others have said, place a pole on the ground after the jump so the horse has to stay balanced and keep the impulsion up until it is well and truly over the jump, you could also try half halting before he jump to regain contact and tell the horse to settle but keep the pace you want.
Hope this helps
Use a placement pole after the jump so he has to look up over the fence and slow down to pay attention also just in general try tricky lines and stridings etc so he thinks and listens to you instead of taking off thinking he knows best.
Try using a polémique on the ground, 1 stride before the fence.
You could also try riding him with your hands held high and teach him to slow down when you raise them
Try to use pole on the ground after the jumps so he can not lose his ballance and has to pay attention. Also if he rush too much try to stop the horse few strides before the jump in first 2-3 times. Then try softly trot walk stop transaction after the jumps. You will ask him to stop not slow down, like that he will understand to wait for your order.
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