Puppy,will changing food make a difference?

My boxer pup is 11 weeks old At 8 weeks he weighed 6.9 ,underweight to be sure,at 10 weeks he saw vet for diarrhea and some blood in stool. Weighed 10.3 . Vet said he was underweight { appt. with vet 9/3/2010 will tell my concerns about weight}. Negative for worms.Did yeast test on ears,negative.He is active alert, playful. Appetite is fantastic.Is fed Blue Buffalo.Looking down on him he still very skinny Hips bony.Ribs showing.Getting taller and gangly.Could this be genetic?Food change? Parents are of appropriate weight.