My small boy like to play/touch it!
Face the problem directly.
We use some books as media to teach the boys how to protect themselves. 是他的私處,不可讓人看.How baby come. We planned to tell them more step by step.
Since the boys are young, we used to tell them this is for 尿尿,not a toy. So far, they seams understand and not touching it so often.
I think you should start to teach him about sex as it is our responsibilities.
Good Luck.