Bulldog bulldozes people and knocks them down

This 9 month old 50 pound + bulldog runs across the yard as fast as he can (and he is FAST) and crashes into the legs of any person who is there, completely flipping them in the air and knocking them down. I am afraid someone is going to get really hurt. Teaching him manners has been a challenge so far although he is beginning to catch on but I haven’t found an answer to this bad behavior yet. How does can I teach him not to bulldoze into people?