I just got off the phone with my BO - he was given Quest in September, and Bimectin this time around, which I guess is an Ivermectin, which might explain the presence of these roundworms.
I'll call my vet in the morning just to make sure what exactly I should use, and their advice on sending in samples, but I'll probably dose him with either Panacur or Strongid just to make sure to get the roundworms this time around.
I guess this is a lesson to always check what exactly your BO is using for dewormer!
& I do realize all horses have parasites, and he is keeping on weight/has a decent coat/etc., however, I think i'm just a little hypersensitive to his worm-load due to his previous history of not being regularly dewormed. I wouldn't be able to forgive myself if I wasn't deworming him properly and he colic'd due to a high worm count.