What is the best way too lose weight?

my husband is overweight and has diabetes. he don't eat pies or cakes etc, he drinks diet soda drinks alot of black coffee he walks alot we have an upstair apartment very high up there about 20 steps he climbs up and down them many times a day yet he is still over weight he weighs over 300lbs for 3 years he eats meats, potatoes and veggies and salads we can walk up to 3 miles yet his weight stays he is on insulin and metformin for his diabetes. he is on other meds for mental health problems. and on meds for high bloodpressure i need advice on what i can do too get his weight down thank you.