"Since there is only the NY address on the W-2 do I claim him a NY resident and don't even file a S.C return,"

You will need to look at the residency requirements for both states. (I do not know them for either.)

New York may argue that he spent more time in NY and therefore is a NY Resident. SC may argue that he is registered to vote, and carries a SC Drivers license, which would make him a resident there...again these are hypothetical's which you need to research.

I guess the question would be - is there any difference in the tax paid to either state, in the following filing situations.

 - NY Resident tax return, no SC tax return (if non resident of SC, with no SC income)

 - NY Nonresident tax return, SC resident tax return with credit for tax paid to NY.

If there is no difference, I would just pick one and move on.