Canadian kid travelling in USA with out passport ,Does he need one to Flight within USA

The problem is that he evidently doesn't have one of the acceptable forms of ID (http://www.tsa.gov/travelers/airtravel/acceptable_documents.shtm) which must be shown to board an aircraft. But the link does imply that if he is under 18 (and traveling with an adult, presumably) he doesn't need ID.

How did the child enter US? Even new born baby needs a passport to travel abroad.

Yes - he will need a passport to get into the US. Once in the US he can board a domestic flight using any of the ID forms that AK lawyer referenced, unless he is under 18. Children under 18 are not required to show ID for domestic flights (even if traveling alone), but this will subject him to extra scrutiny and it is obviously a good idea to have ID with you whenever you travel. If he wants to board any international flights - including to Canada - he will need a passport even if he is under 18.

... Children under 18 are not required to show ID for domestic flights ...

OK. But how does he prove that he is under 18 w/o some form of ID?

OK. But how does he prove that he is under 18 w/o some form of ID?

Good question - the TSA site doesn't say. I would guess that if you claim to be under 18 without ID they take you into a back room and do a very thorough search.