Take him to the park on a 20 or 50 ft lead and let it drag.
Call him every few minutes and every time he comes, he gets a treat, not a normal one but really special treat. When he comes reliably, take the lead off, but still give treats. This is a LONG process (months).

Once he's coming constantly, stop giving the treats occasionly but really praise and cuddle. Now he won't know which time he'll get the treat and which time he won't.

Start using NILIF, "nothing in life is free". It basically means that your dog has to earn everything he gets, from food to going outside. Either sit, lay down, shake a paw, something has to be done, to earn everything. This reaffirms your role as alpha. Google it, this stuff WORKS.

I'm working right now with Misty at the park with ignoring dogs. Her reward is her ball. If the dogs are around her barking (she's not allowed to bark at me) she must lay down and be quiet ignoring the dogs and focusing on me. It is working, but it has taken a couple of months.

Where in Canada are you? In the west, we have a brand of food and treats made by NRG. The treats are to die for!!!!!! my dogs will do anything for one.

Good luck!