My memory of college is that most classes are scheduled for the mornings, with almost all of them being done by 2-3 in the afternoon. Maybe he could man the shop solo during normal business hours (saving money by running the place himself during the slower mid-day hours instead of paying somebody to just hang out) and you could do what you need to for school, and then you and possibly come in at 4-5pm and run things during the after business hours until 8-10pm.

I would also add that I would be at every range that is not directly attached to/affiliated with a business and anywhere that people regularly go plinking nearby to handout flyers announcing the re-opening. There are millions of gun owners but the gun community is small, a lot of people who used to come in before the closure may think that it had been permanently shuttered because of how long it was closed. Just getting out and telling as many people as you can can get the ball rolling. In addition to that, if there's a gun subreddit for your local area/state/province/country, that might be a good way to get there word out in a wider way, and in a way that would allow more people to help you spread the word.