I usually see the infinitive as you mentioned. I think the use of "will" in that heading is to emphasize the certainty of his return.
I haven't followed that story, but I'm guessing that over the previous few days or weeks, there has been debate over whether he would return. So the current headline uses "will" to show that he definitely will return, despite whatever reservations he had.
For headline writing, you could try searching around online. I quickly checked the Associated Press Stylebook, but didn't see too much about headlines (the guide has just a small bit of info about capitalization and punctuation). So I'd look elsewhere.