My 9 month old boxer lab has what looks like bumps all over his shoulders sides and legs... Why?

Just today, he came in from a potty break, and I noticed he had what looked like rough spots in his fur, so I pulled out the brush and started brushing him, only to discover the spots were actually bumps. They don't seem to be skin-related, that I can tell, and when he lays down, they smooth out and aren't visible. But, standing, he looks all bumpy! He is a boxer/lab mix, and he plays very aggressively with his 9 month old malamute mix friend, so I thought he was simply getting roughed up... Now he looks like he's covered in what can only be compared to swollen nodes... What is goin on with my boy? He's otherwise healthy, eats well, drinks plenty, has alot of energy... He's been scratching a little, but has no obvious cause (i.e. no fleas or bugs), and only does it occasionally.