LOL! I felt the same way. My oldest never really did it, but my youngest, who is almost 4, is very much like your son. I just explain to him that it isn't a toy and shouldn't be played with. It is for peepee only. I know this sounds silly, but one day he was just having a field day and I kept telling him it wasn't a toy. Then I asked him if he could buy one at Target - of course he said No, so I again told him - so it isn't a toy and don't play with it. For some reason that stuck and when I see him I will mention Target and he quickly stops. :-) Stupid I know, but it seemed to work.
Good luck! They are boys and I suppose that's what they do.