if its running fine everywhere except for putting the pedal down, id be looking at fuel.
running stupidly rich, esp accel enrichment, can cause major bog down and loss of power upon acel. running in limp mode? check diags...
also: what rpm is he shifting into?
also check the usuals - spark, timing, etc
infact pulling the plugds might give him some idea as to whats going on in the cyls too. ie rich/lean/hot/oil/bhg etc