All dogs should be taught to a whistle. It's so easy and so very effective, once taught properly. Remember, your dog is not going to come to you just because you have one!
Here's how to do it. From day one, before I plonk that food down on the ground in front of him, I blow a dog whistle three times. If you do this every meal he will very quickly learn that whistle means dinner. This is a powerful tool and it is the first step to teaching your pup excellent recall of which will be the envy of dog owners in the park. Over time the noise of the whistle becomes the sign that food is coming.
Next you progress to the garden after perhaps 3-4wks. When he is distracted, blow the whistle. He will look at you, head cocked, wondering "dinner time?!". When he looks at you, you mark it with a big GOOOOD BOY and throw 2-3 pieces of sausage at your feet. He will SPRINT up to the get the saussies. You say another good boy. He will wait to see if there's any more. You tell him to sit, put his lead on and give him another treat. Take a few steps and release him to play some more. Repeat this for another month. He will learn that the whistle actually means food reward but also that while he may go on the lead for a bit he will get off it again, not to be worried about it.
Once this is firmly established you increase the distraction in the garden. Have kids running around, trying to distract him, balls flying around. This is level 3! Once you have him ignoring distractions like this you are free to progress your training to the park where the distraction level gets so much higher. Find a quiet place, using your treats initally and then move towards busier spots.
Remember, come off those treats as soon as possible. Treats are a reward, not a bribe. But never stop them completely. Give him the odd one now again. Pay out like a fruit machine, rarely, it's a powerful way to hook them to their new trick.