Not sure, just guessin from what i've heard in da' past.
Might want ta' chek out his Catilytic (however u spell it) converter. If its busted up from speed bumps or somin, and all the inner wall shit fell off, then that could be the problem. My grandmothers older Ford Taurus had that problem, and couldnt do more than bout 50 mph, she needed a new catylitic converter..
Might want ta' chek that out and also get a second opinion tho. Hope i helped a lil -
