I've been taking my Aussie on mountain bike rides and it seems like on the way back down, even though I'm descending slow for his sake, he seems to tear his pads up almost every time. The tears don't bleed, but as evidenced by him favoring them they're very tender as the outermost layers rip off in one big flap. What can I do to prevent this in the future? Any decent dog "shoe" products?

There are doggie boots made. But he may never forgive you for making him wear them.

Not being a dog owner (presently), my experience with them has been with some friends who train their sled dogs in the summertime in Los Angeles. They have a 3-wheeled dog "sled" that the dogs pull. They used these boots on one of their dogs who had a similar problem to the OP's.

I use Ruff Wear boots for my border collie that has the same problem. Great product and will last a lifetime!