“My adult son is obese and will not leave the house?”

Lack of stimulation easily make people isolated under stable. Seek some doable activities for him

Please seek counseling in order to learn to set appropriate boundaries with your son. As difficult as it may be, you need to look at your part in how your son's life turned out. What type of lifestyle have you modeled for him? Who purchased (and continues to purchase) the foods he learned to love as a child Who has allowed him to live a sedentary lifestyle? Who continues to provide him with a roof over his head and food to eat, despite his unwillingness to get a job? Do you also pay for his car, cable TV, gas, and food? If any of these are true, why would he change anything? He's got it made (in his head, anyway)... In reality, he's been crippled by whatever enabling has gone on for the last 27 years. I apologize if that sounds harsh, but the truth is what his enablers need to hear, as well as ways to stop the enabling, which can only be done by a professional. Neither the enabled (your son) nor the enablers are bad people; rather, they've got unhealthy coping and relating skills. There is hope, but it will take a lot of work and it will require both you and your son to do things that you find difficult and/or uncomfortable.
I wish you the very best. Please don't hesitate...find a therapist TODAY and get started on the path to recovery.

See if this link helps him get moving in the right direction.
https://thoughtcatalog.com/wayne-wax/2014/05/the-5-best-reasons-to-get-fat/

Also take a look at this site for some thoughts.
http://dailyhealthlifestyles.com/20-facts-about-obesity/?utm_source=obesity&utm_medium=%E2%80%8Bobesityfacts&utm_campaign=bing

Have him see a MH professional specializing in dealing with obesity.