
I would recommend a longline for your guy - it will allow you to correct him for ignoring you calling him. Every time he gets away with it right now, it reinforces the fact that he can - and he'll get worse! Have you tried using tasty treats to get him to return? I find tinned ham or cheese works very well, hotdogs too. If you cut them up small you can do a lot of training.
I got my longline from B&Q, I find that many pet shops near me are a bit clueless :) Mine is cotton webbing, about 2cm wide, you can get it on a blue plastic thingy (kind of like for a kite), it comes in 50ft lengths - or loose from the rolls. How much you need depends on the dog - for my dobe, I have 100ft, but I'm going to get another 100ft - he is a fast dog, and likes to exlpore far away from me. Ditto for my other dog. The 50ft lengths there are £5.38, btw!! If you use one, leave it on the ground next to you and step on it when he ignores you - if you pick it up he may well connect you with it, and get wise to what you are doing, then ignore you when he's off the longline again. If you tie knots in it every 10ft or so, they catch under your shoe and help you stop him - good for big strong dogs! Oh, and if he runs fast, I'd invest in a shock absorber - pets at home do them - it'll protect him from damage when you step on the line and he is stopped.
I would do some basic training without distractions at the moment if you can - either take him to the park when there won't be dogs around, or just the odd one that you can keep well away from, or take him somewhere quiet and fairly boring :) When he's reliable there, start uppnig the distractions - take him to the park at a busier time, but stay away from other dogs, then start moving closer.
When it comes down to it, it's all about repetition - the more times he comes and gets rewarded, the better his recall will be. Use his favourite things to get him back - my rott X's recall improved dramatically simply by me gonig crazy when she came toward me - high pitched voice, woohoo!s, arms waving, the works!! and ham, of course :)
Work on him at home as well - try to get him totally reliable there, it should improve him outside then as well. For now, outside, I'd keep him on-leash or longline until he's reliable - or only let him off when there aren't any other dogs around to distract him.