I'm sorry. 😞 That's a long time for him to have lived with you as an adult. Have you had discussions with him before about moving out?
Does he have the means of moving out? A stable job with sufficient income?
Are you also providing care for his child?
Do you charge him rent now, or some form of monthly charge for the resources he and his little family use?
The best way, of course, is to rip off the bandaid and just tell him you need him to move out by a certain date. You don't even have to give reasons why. But some reasons could include expenses, loss of room in your own house from his growing family, too much noise, risk of illness (little kids bring illness into the home), your or your wife's health concerns (if you have them). I'm sure there are more, but those are my first thoughts.
If it makes it easier somehow, and you can afford it, offering to pay some expenses for a certain period of time (like child care costs or a contribution to rent or mortgage) might incentivize him to move out. Don't offer help indefinitely.
Or, a dramatic choice would be to move yourself, and tell him he and his family cannot move with you.
