Sit deep, relax, and breathe
Horses really feel you... Also, circle, circle, circle! If he speeds up, do small circles (bend his nose in, ask for collection, apply your inside leg to bend around you) until he slows down. Then come out of it. If he speeds up again, complete this process each time. He'll start to realize that each time he speeds up, he's going to have to circle, and horses do get bored easily!