What exercises would you recommend to make my horse more sensitive to the leg?

I like to practice the figure 8 . Helps soften the mouth and good on improving leg signals.
He may have become dull from the legs being used all the time to support his energy.
Use the legs a little and then apply a crescendo of pressure with the whip where the leg is. Starting from a feather touch and increasing every second that goes by. Once he moves forward you quit and stop him and let him rest. The rest and reward is the most important part.
Also if he is a slow horse, ask him to speed up ( like at trot) and when he does you slow him back down. Then repeat and slowly increase the time where he's going faster.

This way he understands that if he listenes to you, you will do what he wants to do in return.
I think transitions and rhythm change's can help your horse be more sensitive. And also work your leg. I consider that use always spurs is not a good way, for some exercises helps you a lot, but if you always uses it, finally you'll get a "heavy horse".
Lots of transitions where you do the downward mostly with your seat, and the upward with VERY light contact and a good solid squeeze with your legs.

Maybe give rubber roller spurs a try. I use them and love them!
Well do you believe in voice commands, most horses get up to you clicking your tongue. All I can think of you doing is maybe squeezing with your leg and clicking your tongue until your horse notices that, that means the aid to get up.
In general, try to use your leg as sparingly as possible; get your horse ahead of the leg so they can carry you foreword.
Something else you can do is squeeze to ask for the trot, and if they don't trot immediately then sharply kick. Then when they trot, bring them back to the walk and ask again with a squeeze. They should respond much quicker.
If they don't respond when you kick, use a crop instead and do the same thing, bringing them back to the walk and squeezing again.
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