Yep, start at home..........if he is doing something that he shouldn't be doing, then tell him to stop, if he doesn't, give him a time out.....or take away whatever it was you told him to stop.....if possible.
This can be things that you work on............every day....all the time.......he needs to learn that you mean business.......so, you have to stick to your guns and let him know that you do.............
I would take him to the park, and tell him on the way, that you are going to the park for an hour........take him some treats, and something to play with.........tell him after an hour, if he is good you will leave, if not, he will get a time out when he gets home............now obviously, if something happens, this will change..........If after 20 minutes, he wants to go.....pull him off to the side and tell him he has two choices, to sit and eat his treats, then go back and play, or get a time out at the park..........and if so, do it......Time to start enforcing what you have told him......and hour might be too long, but you get the idea.....and it's not really too long for a child his age..........
You might also try putting him in a stroller......and pushing them both through the park. This way, you're out and about, and he's contained!!
But this all starts at home.......so hang in there, and don't let him dictate your life this summer......he's old enough now to start learning..........
Good Luck and take care.