I have not read all the replies so if I repeat something I am sorrry :)
It sounds to me like he needs his breakover brought back... what does your farrier say??
Can you get us some ground level pictures of his feet?? Also looking straight on the bottom and "down" the heel when you are holding the hoof relaxed by the cannon??
I am betting he has long toes and most likely underran heels that is normally the most common issue I have seen when dealing with horses pulling shoes
I have been called the NSC Nazi more then once ... I hate traditional feed methods of loading our horses up on grains and junk food :)