bought a new pc for my son it has skyrim preloaded.it plays fine ive tested a wee bit.my son wants the creation kit to mod to the extreme.my question is.is the copy genuine and when he gets a steam account can he just dl creation kit for nowt?

I'm not entirely sure of your question but if it's "Is the Construction Kit free if I have Skyrim?" Then the Answer is yes.

If that copy of Skyrim doesn't require Steam to start up, then yes you have been slipped a pirated copy.

No, it's not genuine, you need a Steam account to launch the genuine game to begin with.