How do I teach my puppy to come back?

[Hez]

My 6 month springer Spaniel is not too good at coming back and I as wondering whether anyone had any suggestions. At present I blow a whistle every time I feed him; so normally he is quite reponsive to this when he is of lead... but not always. I call enthusiastically and praise him. I have been practising shuttle recall in the fields near us and he has the hang of this. Its just when he gets distracted by a bird or something that he doesn't return the first time I call him. Its a bit of a controversiall isssue because my dad thinks I shouldn't let him of lead but my mum thinks if I don't he will never learn if he doesn't practise.
I am unsure of what the best approach would be, so if anyone can help me on this I would be grateful.
Thanks,