I would recommed he find a grassy or dirt hill and walk it several times a week for about two weeks to get some conditioning and then start running it. Combine this with some jump roping and some push-ups, squats and core work and he’s got a gym-free work that works.
Dr. Berardi also has a “Keep It Simple, Stupid” version in book form called “The Metabolism Advantage.” Includes diet and a great workout program that would be a TERRIFIC way for your dad to get back into things. I really cannot recommend it enough – around 17.00
Amazon link here:
The Metabolism Advantage: An 8-Week Program to Rev Up Your Body’s Fat-Burning Machine—At Any Age