> He's not going to be a Stud so i am considering castration - but am aware opinions differ on this?
Castration will not cure humping, especially as the humping in this case is due to excitement rather than sex-drive. My last dog was castrated at about 6yrs old, he was a humping-addict, many times he would give my hubby an early morning wake-up call by humping him

castration didn't alter his urge to hump as he humped for fun, LOL. Once we realised castration wasn't the cure, we worked on training him out of the habbit, it took a while but we got there (we got him when he was 4yrs old, he already had the humping-habbit firmly ingrained).
My current dog is entire, there is no way he will ever be used as a stud, but he is keeping his 'bits' :) He is a total pain on walks at the moment as he is obviously picking up the scent of a bitch, but training is helping that allready :)
>Is there harm that can be done and, if so, what?
The down-side of castration can include, undesirable behaviours such as agression/fearfullness/sexual behaviour. Early castraition can effect bown growth & even stop maturity, leaving the dog in a constant puppy-like phase. Castration can also cause your dog to become a target for other dogs to hump.