I have a 3 year old GSD male who failed his first OFA hip exam, mostly because of poor positioning in the xray. Is walking good to help strengthen his hips?

I think your best chance of building him up in the proper way would be to find a canine fitness trainer or sports medicine/rehab vet. Depending on your dog's structure and joint conformation, they can come up with an exercise plan for him to build muscle in the appropriate areas. Using a weighted vest may help but may not be the right type of exercise for the muscle building he needs. I have a rehab vet about 1 1/2 hours from me and she's been very valuable in helping me build my dog's muscle tone and fitness.