I would have to see pictures to comment on this specific horse.
Horses with the LP gene (the "on off switch" for Appaloosa characteristics, and the expression of patterning, if any is present) and/or Appaloosa patterning, can go through weird changes in both base color and patterning.
I had a horse who was born red dun with a blanket over his back and hips. As a young stallion he had alot of white speckling in his red dun forparts, down his legs and etc. A few years later this reversed and his red dun areas became more solid again. Then as an aged stallion the white speckling returned. He never really roaned, as in, gained white hairs that were dispersed/blended througout his dark haired areas..... but the amount of visible white outside his blanket increased, decreased, then increased again. If he had lived longer it may have decreased again.
The presence of LP can indeed interact with base color to cause changes. For example, my current stallion was born bay with a small white blanket over his hips. He roaned and is now mostly white. He does have color on his legs, mane and tail, and over the "bony prominences" of his body, but he no longer has any black points-- areas that used to be black look dark chestnut-- but he IS still genetically a bay.
There are at least two Appaloosas I know of who look dun or palomino, but who have been genetically tested as black. At least one had base colored areas that were the typical color of a foal who was going to be black, but those areas shed out gold.
It is also possible that the winter coat was lighter and faded to almost white, and when the horse shed again, he was again dark. This is not common in chestnuts, but very common in dilute colors such as palomino or buckskin. It seems more extreme in horses with LP.
Horses with LP as well as the grey gene sometimes do go grey very quickly, sometimes shedding out totally white on their first foal shed.
If the Appaloosa in question truly greyed, as in, from the action of the grey gene (which is unrelated to Appaloosa coloring) I agree he would not gain color again later (except maybe for flea bites, which do appear as little "re-colored" specks in greys who have already turned off mostly white). However if his "going white" was due to the Action of LP, I would not rule out re-coloring later on-- its not common, but based on the Appaloosa weirdness I have seen, its not totally impossible.