If he was mine I'd cut out throwing things for him on walks. The chasing, swerving, turning and jumping in the air when dogs chase balls or anything else can do a lot of damage, especially to a young pup whose growth plates are still open and whose bone density isn't fully developed yet.
Another reason to limit it or stop it is that he may become overstimulated and then not be able to relax when you get home. A bit like taking small children to an adventure playground and hoping they'll be relaxed as soon as they come off the equipment there! It might be the reason why he zooms round the back garden after walks.
Could you try either stopping or limiting the throwing and making sure that his off lead time mainly consists of him having the opportunity to sniff, mooch around and just be a dog? Maybe do some training too, but always end with some just being a dog time to encourage him to relax.