I know about how often to deworm and how to do it, but should I deworm before or after a workout? Before, after, or in-between his meals, etc.?

The first photo is a photo of my boy a few days before I got him and the second photo is of him about two months after I got him. His ribs still show but he's an old man so I don't think that will go away. I feed him equine senior, alfalfa and coastal hay everyday. I add a little oil for extra...