Once again everyone thanks for your replies.
As my emotions are all over the place at the moment, I guess I'm just looking for an easy option to lay blame. After reading all of your comments, I have realised that yes, it MAY be hereditary or it MAY have been caused. Hopefully the specialist will shed more light....
Our boy has grown up with a staffy, they are always having a play and a rough and tumble and often have visits from our friends dogs. Maybe it was caused then, who knows. One thing is certain, our dogs have excellent diets, varied between Eukanuba and Hills Science diet, they never eat any old junk and get our off cuts of steak and chicken. In fact sometimes they eat better than us!
The reality of it all is he's young and it will effect his life from this day forward. It's sad, upsetting, heartbreaking and we will do all we can for him to lead a happy, dignified and healthy life as I'm sure everyone on this site would do.
thanks, cheers
have a great weekend.