Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that this little one is having so much trouble.
HOD is possible, but he is pretty young for this to be the cause of his symptoms and that really wouldn't explain his gas and indigestion. If that is the cause slowing growth by feeding limited amounts of a high quality puppy food may help. He is very young to try to feed an adult diet and I would worry about brain, organ and gastrointestinal development with being fed an adult food. Pain is controlled with nonsteroidal anti-inflammatories.
I also find it strange he doesn't seem painful. HOD is very painful for these pups, and most of their loss of appetite or difficulty eating is due to pain.
I would be concerned that he has an X-linked form of muscular dystrophy because he is a male and with his range of symptoms. This is an X-linked inherited disease, thus we see signs in males. Males show muscle stiffness, difficulty eating, and weakness at an early age, which can progress to a flat footed stance and muscle atrophy as the animal gets older. Diagnosis is highly suspect when we see initial high serum levels of CK and confirmed by demonstration of hyalinized and mineralized muscle fibers on biopsy. Prognosis is guarded to poor because they continue to worsen as time goes on and we don't have any effective treatments.
Another possibility would be Labrador Retriever myopathy. This is an autosomal recessive defect (so both sexes can be affected) and causes a stiff gait and marked muscle atrophy in puppies of both sexes. Signs worsen with cold, stress, or exercise, and affected dogs may not be able to keep their heads elevated in a normal position. This may become apparent as early as 3 monyhs of age. Tendon reflexes are usually absent. There is no treatment for this disease but the pup's prognosis is favorable, so affected dogs can make good pets, because they often stabilize by 6 months or so of age. Diagnosis is via a muscle biopsy.
I think your pup needs further testing.
Please let me know if you have any further questions.