Forget time travel. No sling-shotting. No Guardian of Forever. No temporal rifts to slip through.
Once again... if he lets it slip that he's from the future, unscrupulous elements could easily take advantage of him.
The only thing more I can say, is go read Engines of Destiny
by the late Gene DeWeese. It's about how wrong best intentions can go when Scott beams Kirk away right before he "dies" aboard the Enterprise-B.