I just got an English Mastiff (14 weeks old) and his front legs have a slight bowlegged appearance. The vet said it may not be a problem but it is too soon too tell. Does anyone have any experience w/ this?

This could be Panosteitis. This can be a problem in large and giant breed puppies, but can occur in medium sized breeds as well.

Pansteitis (Pano) is commonly thought to occur due to improper growth rate from overfeeding and improper calcium/phosphorus ratio's.

Here's some information:
P - Panosteitis

Hope this helps