As a big man myself, 6'3" and 380lbs, I have a tough time with sleeping bags myself. I have had the best luck with long bags that I have used as quilts. Typically I will put my feet in the footbox and zip it up to about my knees or so and then drape the rest over me like a blanket. Combine this with good layer of clothing and he should be pretty good!
Also, now that I hammock camp I just pull the footbox over the end of my ridge rest pad and it serves the dual purpose of keeping my pad and bag in place.