My Sentret likes to climb up into the roof. How do I get him to stop? Can I put a camera up there so he can still monitor the area?

You could! Don’t forget also to remove any climbable object or surface that allows him to climb up to the roof if you can or (if you can’t) at least cover or block his access to them. Additionally, train him to stay inside or stay within a certain area of your yard, using a combination of positive reinforcement and liberal use of the heel command.

Or alternatively, if all else fails, return him to his poké ball whenever you spot him on the roof or preparing to climb to it. Eventually, he’ll get the idea. If he’s anything like most sentret, anyway.