I have one mouse (female) as a pet. She was my best breeder when I had a colony, so she was saved to live out her life in luxury!!

She lives alone in a 10 gallon tank. I have a exercise wheel and some cardboard tubes in there. I also have a few wooden bird toys hanging from the screen top. She chews and climbs on these quite frequently. She has been there alone for a few months now, and seems fine.
Be careful male mice's urine is quite smelly...use the vanilla extract.!! Good luck with him.