Can my ex make me take my son back to ohio?

The time sequence is a little unclear.  When you speak of six months, the reference I think you are making is not to "residency" (in Nevada, that is 6 weeks) but UCCJEA child custody Home State status.  For a recap on the rules of such matters, see the article "The Basics of Family Law Jurisdiction" posted at http://willicklawgroup.com/child-custody-and-visitation/, but for a meaningful answer as to what you, or your ex, really can do under the facts of your case, you need to create a detailed time-line of who was where, when, and take all the documents and consult with a family law specialist.