It’s going to be very hard to find a good over/under in that price range, in my opinion.

A used Citori field might be had in that price range. Find one on gunbroker and tell the seller your story. You might find someone willing to work with you. I know I would.

For your specific application, I'd say an older model Citori field grade, or similar gun, with fixed IC/M chokes would be ideal. They are a quality gun. And, they can be had within your price range.

I would, however, suggest you re-think not having an auto-safety. If you are wanting to minimize the number of things he needs to remember to do, I would say remembering to re-engage the safety is definitely something you don't want to have to remember to do. Make "safety on" the default. I am confident that pushing the safety off to shoot will quickly become automatic.

I agree with this. Missing a shot because he forgot to take the safety off is not the end of the world. Maiming or killing someone because he stumbled while carrying a loaded shotgun, in off-safety condition, is the end of the world for someone.