Assuming the video is showing the whole situation, the guy was acting suspiciously. When the police ask you for ID and you can't provide it, most honest people give them a full name and address and say we can go to my house at whatever address and I can get it. Or he had his phone with him and could have put the cop on the phone with somebody at home. Criminals use kids all the time anymore. We've all seen the videos where the parent gets the little kid to go snatch a package off a porch, et al.
But body slamming an obviously unarmed guy before all those options were used, yes excessive. Especially in front of the young boy. The officer should be severely reprimanded if not fired. (And who bothers to carry their billfold/ID when they're just going for a walk around the neighborhood? On the other hand, who is up at the crack of dawn going for a walk with their young kid?)