C.,
I've never been as compelled to respond as I was to your request for advice. I'm infuriated that a teacher would do that to an 8 year old child. That man is a bully. Sorry, but if my son came home and told me a teacher (or anyone) hurt him, you'd better believe I would not waste time writing to anyone, or sending emails, or requesting a conference. I'd confront that individual immediately and demand to know why he/she felt it was necessary to humiliate and hurt my child.
You say PE teachers are supposed to act tough. None of mine ever did. I've seen coaches act tough. Keep in mind though, there is a difference in being tough and being mean.
Now I'm not saying go up there and stand toe to toe to this man, but don't waste time in confronting him. Everyday that goes by is another day you allow him to humiliate or hurt another child, if not your own again.
If it were me, I'd go to the school office and have him called in. Then there infront of other personnel, I'd confront him and make him aware that he has no business EVER putting his hands on my child again. Be calm and polite, not confrontational, but be firm. After confronting him I would ask for a conference with the principal.
Don't worry what the school personnel are going to say about you. Or that you'll be classified as "one of those moms". Instead you should proud to be "one of those moms" that stands up for her child.