At his weight he should be able to Look at a tread mill and loose weight.
In all seriousness are you sure hes not eating good around you and then gorging himself at other times.
Even on a very strict diet his metabolism should not be effetced that much due to the enourmous stores of energy he is carrying. If he needs energy it will tap into all the fat.
As for a blanket equation. Well at that high of weight you really cant go off his body wieght now. I would suggest doing something Like T-Dawg II and use the target/goal weight as the starting point. He should be able to shed weight VERY fast without much worry of losing muscle at first. Then It will slow and down as he gets closer to the goal.
Just my take. I think T-Dawg calls for body weight x 16 then subtract 500. Just ick the goal weight x 16 ad start from there that should have him shedding weigth fairly quickly if he chooses the correct foods and works out.
Hope that helps,