If it's only a 20 minute ride you want surely you can get out in most winter weather. Unless you get a lot of snow & ice where you are.i'm taking it 12boy isn't a fan of exercise. Perhaps he has a history of being teased at school....
A 20 minute ride on an upright bike targets lower back pain very well. I'm looking to do indoor riding on my h type brommie in winter months when i'll be riding far less but working over a pc / desk far more and therefore likely to experience increased episodic lower back pain common to my age group.