I get very frustrated telling him over and over "Sit down." Scared he's going to injure himself when he gets up. I think he must be bored; or else this is part of typical nighttime agitation. But how to deal with it and keep him safe without getting angry at him?
Best of luck
Or even perhaps put the jacket or sweater around the back of the wheelchair and button or zip it in the front?
If this frustrates him, it isn’t fair to him to do it except in very infrequent emergencies, but if he’s just comfortably unable to figure out why he isn’t able to stand up, it may serve as a deterrent for brief periods of time.
Would it work (also temporarily) to push him into a new position every several minutes?
Maybe you could try to take a deep breath and try to calm down a little bit about this. Yes, do what you reasonably can to keep him safe, of course. But life is not without risk so don't drive yourself too crazy with it, if possible. You being frustrated and angry and scared about this is no good for you. It sounds like a lot of stress.
Is there a lap belt in his wheelchair? If he really should not be getting up at all, that might help, as long as you are there to undo it when he does really need to get up.
Good luck.