Leave it in there. He's obviously hungry. Some balls will not eat with people in the room, many users here have said they have to put the mouse down and leave and it'll be gone in a few hours, and they never see them eat. Remember snakes can go a long time without eating, so even if he chooses not to eat, no need to panic. I would just heat the mouse, plop it in there and see if he goes for it. Once you have your scale it'll be easier to tell if he's losing or maintaining weight. Too, if you get really desperate, you can assist feed - though that should be a last resort only and I wouldn't do it this week, I would give him another week at the optimal temps an humidity and try offering like normal in 5 or so days before thinking about assist. (Too you should have a weight track on him to see if he NEEDS assist feeding or if he's maintaining, most people won't try an assist until there's been a long time and weight drop without feeding.)

You're on the right track either way! It's too bad the pet store can't tell you when he last ate. (I wonder if they even keep track...).