The bigger the bone structure the longer it takes to develop. Whippet types take lesser time than say a deerhound, greyhound etc etc. Just because its a big strong pup, doesn't mean his bones are fully developed. My old man knew a few people who's young dogs died, two I think. Running hares too young. Just let it be a pup.If its keen on chasing stuff that moved still doesn't mean anything, my bitch at 4-6 months was well aware anything moving she wanted to chase, dogs off a lead, cats, birds, you name it she wanted to run an run an run at it lol you'll just have a higher chance of injuring it,