I think is time to explain to my 6 year old boy the difference between female body and male body but how to explain and how detailed should it be? For instance, when i hug my boy, he did say "opps sorry, i touch your b****t and he laugh"... how should i react? oh gosh

Be firm in your words. Tell him. I am your mummy, i am ok with you saying such words. But with other people you cannot say such words! Be very firm and tough on it.

I'll take it lightly. and just say that's okay. on another occasion take him to the library and read books about our body.