How do I stop my child from dropping his pants?

[ByKookyCritic]

My 3 year old had began to find it extremely funny to drop his pants and shout 'Willy!' ever since he's been potty trained. We worked on this and I thought it had stopped as he doesn't do it at home anymore but now his creche has told me that he still does it there and still finds it funny and that there has been a complaint made by another parent. They said if he doesn't stop doing it he may be asked to leave. I don't know what to do, I've spoken to him about it but there's only so much he understands and since he doesn't do it at home I'm struggling to think of a way to show him this is not okay? Just feeling a bit blindsided by this all and looking for some advice.

How did you stop it at home and what are they doing to stop it at the creche? You say there's only so much he understands, do you mean he has SEN or just that he doesn't see the problem with what he's doing?

I think its that the other kids think hes funny when he does it. At home we just didnt react, when he pulled them down we would just pull them back up and tell him it wasn't very nice to show himself and honestly it wasn't a huge issue. The creche said when he does it they tell him it's bold but then she began saying they're going to have to talk to their manager about what to do with him and I just went into panic mode. I'm going to make up a list of what I want to say tomorrow and go in and ask to speak to the manager. I've spoken to him this evening and asked did he do it and he said his friend did it as well so I don't think it's just him in it. I'm just a bit lost as to what to do because he's obviously smart enough to know we don't entertain it here and I know it's not up to the creche to parent him but I feel like there's only so much I can do when the problem is only happening under their care?

Just to clarify as well, they mentioned it once about 2 weeks ago and we had a talk to him and he promised not to do it again. So today was the second time they've mentioned it but it was immediatly followed by saying how he would potentially have to be removed from the room if not the school so I'm panicked now trying to figure out how to fix a behaviour I don't see him doing myself before it's too late.