Could give some more specific suggestions if you wanted to send pics of his feet & info about his diet & management.
Basically I would definitely keep him bare & frequently *well* trimmed. I would use hoofboots, possibly with pads as well, rather than shoes, to provide support & protection to the whole foot(rather than just the ground surface of the hoof walls), to allow him to exercise and use his heels comfortably - because that is one huge key factor. The more exercise the better, so long as the horse can be made comfortable to do it and his feet can begin to work correctly. That done, he *should*(always exceptions unfortunately) be on the road to rehabilitation.
A diagnosis of 'navicular' without an xray basically means unexplained heel pain. I don't think there's any less to worry about tho, as to whether or not the horse has reached the point of bone degeneration. It is thought that damage to the navicular bone is most likely caused by toe first landings, causing the whole impact of each pace to meet the top of the navicular bone. This happens when the horse's heels/digital cushions are too weak - often due to lack of use(soft paddocks, high heels...) - to support him comfortably, so he begins to land toe first. Of course this takes the heels further out of use and therefore further from rehabilitation.
The bone deformations that have occurred may or may not become 'repaired'(calcification can be worn away again once the horse & his feet are moving correctly), but that doesn't necessarily mean the horse can't become sound again regardless. I would think a 6yo horse should have a good chance of a sound life with the right 'treatment'. Ie hoofcare & management. Pete Ramey hoof care heals founder in horseís navicular disease farrier
is a great site with heaps of well researched info & I can highly recommend Pete Ramey's book & DVDs too. While as with every subject there are different opinions & approaches, and I don't necessarily agree with some of the details of their approach, I think Treating Founder (Chronic Laminitis) Without Shoes--Home Page
is still a great site to learn from, with heaps of info too. There are many other great sites, with many 'case studies' of lami, 'navicular', etc too.