If I wanted to illustrate how to breed a horse NOT to stay sound, or specifically how to breed for navicular, I'd probably use this horse as an example.
Even if, as someone else suggested, part of the problem is contracted tendons from fast growth and overfeeding, 1.) that's not exactly an argument to buy him. 2.) it makes me concerned about epiphysitis and OCD (osteochrondosis dessicans, not obessive compulsive disorder)
He's the horse people are referring to when they say "You can't ride color."
Run, don't walk, away from this one.