I have little squeaky 'hearts' that I kept from destroyed toys that I can take out with me. The squeaky noise gets them to run to me (some are blow-through and can be very compact).
Have you tried real meat as treats? One of my formerly anorexic basenjis liked desiccated chicken (I never understood the appeal of 'cardboard' like meat to him). Is there anything special that he really likes (attention or toy, etc.) that might be considered 'a treat' to him? To reinforce the recall, a treat can be given every time the toy squeaker is used at home in a controlled environment so that it may still work when you are out at the park. Just save it for "really need the recall" times, though squeaking it when you are back at your car and can safely give him a treat would be a good idea.