His back end looks a bit odd to me, though I can't put my finger on why. I wouldn't say he looks lame on one specific leg though. There's a few moments when one hind or the other isn't extending or flexing freely - in particular, around 1:10-ish really caught my eye because that off-hind really isn't extending forwards the way it should. It looks like he's very stiff at the back.
The sand in your school looks a bit deep and I'd be interested to see him being trotted up in a straight line on a hard surface.