Mild calorie restriction, slowly pulling out the worst foods. Buy him a food scale, and be firm with him about eating only measured amounts of food, and cataloging everything. The biggest thing with huge folks is that they don't have an accurate measure of food amounts, they think their little snack is #300 calories, meanwhile it's 3 times that.
Get him started on some low impact stuff, and hope for the best, it's a long road.