How could my father lose weight with a degenerative spinal condition?

Non-impact exercise. Non-impact exercise such as a stationary bike or recumbent cycle also. Swimming could also be of benefit as well as water aerobics.
Boost Metabolism. I assume you are asking this question - because ur dad have difficulty exercising. You can focus on building metabolism up by switching to fat burning with carnitine supplements, adding natural thyroid boosting supplements like iodine and thyroid glandular support by biotics, supporting pancreas with chromium and adrenal support with herbal and minerals to improve weight loss + omega 3 supplements.