I have an 18 month old who does the same and he gets time outs in the play pen. When he misbehaves I tell him "no thank you" and give him 3 chances to stop & or come if he doesn't listen he gets a time out! Even if we are at the park he goes in his stroller. After 2 weeks of time outs all it takes now is to say want to go on a time out? He usually stops and says "no" LOL.

You need to also make sure his parents are on the same page for disipline as if they don't do it the same it just makes it harder for us!

Good Luck!