My best guess is that it's one of 2 possibilities. A) they need de-wormed, which you already said you did...though if it keeps up and he's still hanging on to that hair in a couple of weeks, I'd have a fecal run on him just to make sure, or B) it's due to the funky weather. All of my horses were late shedding out this year because it would be warm for a while and they'd really start shedding in earnest...then it would snow and be below freezing for a day or two so they'd stop shedding. Then it would warm up and they'd start shedding again....just to have it snow again and be cold.
The very last of my horses to still have winter hair finally shed out the last of it about a week ago.