Best Way to Achieve Massive Weight Loss? Lose 100lbs

My husband is 30 years old and needs to lose over 100lbs (5'10" ~300lbs) in order to be healthy (i.e. high blood pressure, sleep apnea, high cholesterol, etc.). He used to be in excellent shape (pro-soccer player, and later an amateur fighter) but due to injuries from his playing careers, a motorcycle accident (ankle, knee, and shoulder injuries and surgeries) and the subsequent weight gain (severe back pain when on his feet for more than 30 min) he can do very little exercise (esp cardiovascular).

He has tried to lose weight by cutting down on caloric intake, but that is no longer resulting in weight loss and overall the situation is becoming extremely frustrating for both of us. I am in good condition (5'7" 135lbs ~20% body fat) and prepare healthy meals as much as possible but neither I nor he know what else to do. Thank you for your help and suggestions.