Maggie was 8 months when she had her first season.

Zoe - Arnica is great - I use it at the first signs of any lameness and it always seems to work well. I wouldn't worry too much and I certainly wouldn't pay £500 for an x-ray unless I thought it absolutely necessary. Could be just growing pains or he has pulled a muscle - he is still young. Maggie had a lot of lameness during her first 18 months - even was told by a vet once that he thought she may have hip dysplasia as she had been lame on her back leg for a while - I was offered an x-ray but decided to wait. She got better and has never been lame on that leg since. Munsters are a recipe for disaster really - big heavy dogs that run around like lunatics! If I were you I'd rest him for a week - lead walking only, give him some arnica and see if it improves.