He can't turn into a palomino, but he can have some variation in his coat year to year. My cremello is looking darker this year than he did last year- he has a white blaze that wasn't very obvious last year but is very clear this year. In his case, I'm guessing it's diet related (I've had him just over a year now and feed him much differently than his last owner) He's not fully shed out yet, so I'm waiting to see what he looks like once he's completely lost his winter coat!
On a side note, your horse looks perlino to me in that first picture you posted. If his mane is always that much darker than his body and isn't just a trick of the lighting in that photo, I'd bet he's actually perlino. Definitely possible with the buckskin sire.