This is awful. You have to wonder how it is possible for not one, but two, adults to believe this is an okay way to treat a little guy. Calling the police doesn't always help. It depends upon who answers the call. If the officer chooses to investigate the video, call Child Services, and take the boy for examination, it would be great. But I've seen officers call it the witness's word against the parents, and blow it off with the explanation along the lines of "everyone is tired, the kids misbehaved and got a spanking", and that's the end of it.
Maybe today you could contact your local child abuse hotline, explain to them what you saw, and ask for guidance. Perhaps they can pull the tape & get the parents' info.
Honestly, it just makes me wonder why the hell some people have kids. I do not get it.