Be sure he can't get out except the way he came in, if possible. Leave lots of his favorite treats near where he got in - something warm and smelly such as scambled eggs and cheese. They love that. Only odors will lure him out, unless like mine very tame; mine will usually come over to my voice or to a repeated tapping.
You will have to wait and hope he comes out, and then lure him into a deep pail or such which you can turn rightside up once he is in; or, you can order a Havahart (they are online with that name) humane trap. I have used mine many times to catch my
fancy mice. I don't suppose you can't get the trap inside where he is.
Leave out water where he went in.
Good luck.