[anonymously]

It is always best not to give a dog any supplements or over the counter medications to a dog, unless prescribed by a veterinarian that has examined the dog.
BTW: Benadryl is a antihistamine. Every dog is different, these things can have side effects.
Only a Dr can diagnose and prescribe.
It sounds like you have had x-rays done and already consulted a specialist…if not, it may be time for a second opinion.