Same here, my current weight is probably higher than his, but I'm a fairly tall guy (6', 360lbs or thereabouts).
Just by controlling what I eat down to a 2000-2500 calorie diet, I have lost about an inch a month off my waist, and I do stretch (some) and I do exercise (some) and I like to "go hiking" in a lot of the area parks. When I am out and about, I always bring a walking stick and a bottle of water.
He may or may not benefit from all of this, but I find actual walking to be a LOT easier than a treadmill, and better exercise, too. If he has knee problems (My knees need to be rebuilt) then he should definitely consider a supplement like fish oil or glucosamine sulfate (they work for some, others don't really notice a difference, they REALLY made a big difference for my knees), but he should definitely make sure to research anything he takes.
Taking in a regular stroll is great, the situps or other exercises will make a big difference, as well; but ONLY if they help his motivation. If they become something he dreads, then it is probably better to find a different sort of exercise.
One other thing; I am not a professional, but I really do not recommend jumping jacks until his weight comes down, unless his legs are in EXTREMELY good shape. At this weight, they really put a lot of stress on the knees and can cause a lot of wear and tear. One thing that works very well for me as a leg exercise is just doing very light amounts of weight on the exercise machines, with a lot of repetitions, if he has that sort of thing available. Another exercise (which sounds REALLY DUMB) is basically "standing up". When you stand up from a very low chair, at my size, it is a very good muscle-building exercise; no matter how dumb it sounds. If I do it 10-20 times, standing up from a single milk crate (about a 1'x1'x1' box), I am essentially building those leg and knee muscles that are so vital at my size, and I break a solid sweat doing all of that over the course of several minutes.
Like I said, it sounds dumb, but at my size, pretty much anything I do is exercise. The most important thing is to not do anything that doesn't feel good after doing it for a few minutes; both for motivation, and for preventing injury.
