I don't mean to be under-educated about this, if he is using Diesel from the same gas station consistantly; could the fuel be dirty? I filled up a car a few year back at the same time the station was filling their tanks and I guess I got a lot of dirt in my tank. That same day, car would not start. It felt as though it was ging to tear itself appart. Long story short - dirty fuel rails. Mechanic said NEVER fill up when you see a fuel truck filling gas station's tanks.