Aw - I know what you mean - it is maybe a bit of guilt tripping
I tend to just talk to them until they cheer up or walk off and get sick of you talking to them
If he had a nice time out of the cage. He looks very sweet. Did you change anything in his cage while he was out? Although if you did he might be looking mad rather than sad!
Hope you don't mind me asking but the shelf he's on looks to be some kind of grid - although maybe it has clear plastic on top? If it is a grid shelf then that can be very bad for their feet and cause bumblefoot - not sure if you knew that. Easy solution is put a piece of cardboard on top of it (or hemp mat but that's not so easily accessible).
Or give him some extra treats and hide a couple so he has to find them and gets distracted