The law itself is the four ammendment. Dude has a strong case for excessive force. He was down, not a threat, not being violent. He wasn’t kicking the officers, he wasn’t making verbal threats. He wasn’t fleeing. He was definitely not being cooperative, which is a very bad idea, but not something he can be executed for. There is a legal doctrine known as the Graham Factor that determines the legality and practices of police use of force. It’s basically a checklist of very bad things a person needs to do to warrant use of force, especially use of deadly force, such as pressing a gun to the back of someone’s head and threatening to kill them because they are making your job harder. This dude meets some of the parts of the checklist, but not enough to keep this from being excessive force on the part of the officer.