D.P.
This will seem strange, but I'll tell you from my own experience it DOES work. I have a son who was receiving early intervention for a speech delay, and one thing that was emphasized was time limits. Time limits help to transition kids from one activity to another. Try it at home first. When he is playing with a toy that he likes tell him "one more minute, and we will be all done with the toy." Then begin something new. Play with something else, or have a snack. If he's in the tub say "one more minute and we will be all done". Then drain the tub. You can carry this concept over to the park. If you are consistant it should really help. I know it sounds a little lame, but it works.