C.H.
Hi,
my son's friend at the playground used to be like that. He stopped now after his mom's consistent and diligent way of dealing with it
First time he does it, he gets a warning
Second time, he goes to a time-out at the corner of the park
Thirt time, he goes home.
A few times I saw my friend took her son home despite the heart-wrentching crying and protest, but the next day he would behave so much better. Eventually he got the message. He was about 2 at the time. I don't know if 18 months is too young, you would have to decide. I do know that it must be hard on you, because you know he is a good boy but others may not understand that. So I would say give this method a try and see if it works. If not, maybe he would outgrow this phase. Hang in there.