Even if it is Navicular, with proper care, he might still be able to make someone a really nice horse, though I probably wouldn't want to do eventing with him. We had a horse that continued to be sound even under a relatively heavy workload for almost 20 years in spite of having Navicular. He was probably the exception to the rule, but where there's one, there are more.
IME, horses with Navicular only begin to walk differently than normal when their degeneration gets to a certain point.
Don't give up hope on him yet.