First off, foal colors can be almost meaningless when it comes to the eventual adult color-- many foals are born looking one way and then shed off in an unexpected direction.
Were either of his "sorrel" pony parents red with a flaxen mane and tail? If so, they might have been a bay with the silver gene (commonly called silver dapple, taffy, etc). Silver dilutes black pigment and leaves the red pigment alone, so when a bay has the silver gene, the body stays red, but the black mane and tail and legs are much lighter.
Although there are usually some visual "clues", a silver bay can be pretty easily mistaken for a flaxen chestnut/sorrel. Silver is pretty common is Shetlands, and some other pony breeds, and also in Miniatures.
If either of his parents were actually a silver bay, it would explain how they were able to produce a black foal-- the bay contributed the E for black pigment, and the foal didn't inherit an "A" for restricting black to the points, nor did it inherit silver for diluting any black which was present.
Here are a few silver bays (pinto and solid), for reference--
Another possibility is that he is actually a very very dark liver chestnut. He looks black to me, but there are some chestnuts so dark that they look very black.
Depending on what his parents looked like, I think the silver/bay parent explanation is more likely, as he does look like a "real" black....