I do not think that his father is abusive. I think he got a spanking. It was probably hard enough to hurt, but left no marks. I would not consider reporting this. I only question if I should bring it up with mom or let it go for this time.
I know some people spank. I just find it hard to teach a 2 year old that hitting is wrong...that's not how we solve problems when the people they look up to solve problems through hitting. But, for now he's a good little boy and I have to presume that this was an isolated incident. I just don't like it.