President Obama could certainly pardon him without any qualifications. Which is not to say that some people in government wouldn't use their bureaucratic powers to harass him once he returns because they dislike what he did. For example, he might get the run-around when renewing his driver's license: "Oh, we need to see your prescription from your optician before we can put your eyeglass restriction on your license" (obviously not true but he won't get his license until he complies)