He is known for his martial arts skills and was trained by the famous stunt coordinator, Jeff Imada, who worked on "Kickin' It". Howard has a black belt in Taekwondo and is trained in various other martial arts. He's comfortable with a lot of the action sequences, especially the ones involving kicks and punches.
However, for the more dangerous stunts, like falls or high-impact maneuvers, a stunt double was used. Safety is paramount, especially when working with younger actors. So, while Leo did a lot of the action himself, the production team would use a double for the more risky stunts.
It's a good example of how stunt teams balance the excitement of action with the safety of their actors.