Seems like a couple of bored cops and that they've had too much trouble with kids lately. As a State Troopers daughter I've heard both sides of the story.
Unfortunately with so much violence these days, it's not unheard of for cops to be heavy handed. Doesn't sound justified but we're only hearing one side of the story.
I don't agree that you believe your kid can do no wrong, but both sides of the story need to be told. Then appropriate action taken.
I'm in my 40's and I get nervous when a cop even comes up behind me, let alone if they pulled me over. Everyone reacts differently to stressful situations, nervous, laughing, crying etc. I put no stake in how people react in a stressful situation as we're all different.
I'd definitely get a copy of the tape, listen to both sides and take it from there.
My best to you and your son and I hope this works out and doesn't sour your outlook on cops as they really are there to help us.