Lab Puppy illness

Question
8 week old choc lab male puppy. His symptoms are: distended belly after being dewormed 3 times, 2, 4, 8 weeks (Nemex twice and a three day course of Panacur.)  Weakness in rear legs, bow legged. Less energy than the rest of litter, smaller by 3 lbs than littermates, some GI problems(constipation and diarrhea) when younger but normal stool now.  Very hungry and thirsty but looks very thin.  Normal bloodwork at 4 and 10 days old but no further bloodwork done.  X-ray and ultrasound were normal.  The rest of PE was normal although one vet noted "enlarged stifles".  Any ideas?  I am taking him back to the vets tomorrow for a blood glucose test to rule out juvenile diabetes.

Answer
I am afraid this is beyond my expertise. Since I am not a vet and this is a very complicated case, I feel that only a vet should comment on this.
If you bought this dog from a breeder I would certainly let them know the expense you have incurred for this puppy. It sounds like a congenital problem and that is even worse.
What kind of food is this puppy on? Has he been tested for Lymes?
I really think you should re-submit this to one of the vets on this board here.
Let me know what you find out.