While there is always a remote possibility that something more dire is happening, generally, it makes sense to assume horses, rather than Zebras, when you hear hoof beats. When the toes get too long it slows the breakover time so he will drag them more while rolling over them during the stride. That will abrade the front more, shortening the toe length. The underrun heels contribute as they tilt the foot back, lowering the palmer angle.
Underrun heels are when the heel buttresses are are located further forward than they should be. It means the weight of the horse is actually coming down behind the heels. If you've ever had your foot slip part way out of your shoe , then taken a step, your heel lands behind the back of the shoe rather than in it. Not comfortable, nor healthy, if you continue to walk that way. You also need to add some height to the back half of the feet relative to the front half in order to tilt the feet into a more correct configuration. SO to correct the underrun heels , you can't just rasp them more to move them further back immediately as that will make the already compromised palmer angles worse.
There appear to be knife and/or rasp marks on the sole. I would recommend never taking anything off his soles again as there doesn't appear to be any concavity there, so likely the soles are already too thin.
Again, need a full set of labeled photos to be more specific.
Have you ever had any rads done of his feet?