With the upright foot, my horse had the exact same thing. With following the live/sensitive/functional sole, and keeping a short cycle, 6 weeks or less, it solved itself in under a year. Her heel still grows faster on the left, and underruns on the right, but the short schedule keeps it in check, and now it's real hard to distinguish, her angles are off by 2 degrees, which is nothing. When I got her, it was an 8 degree difference. Just continuing proper trimming guided by the live sole let her correct herself.
I don't see anything that continued proper trimming would not fix. And careful supple-ing exercises to help him travel more even both ways