What can I give to my 3 year old child to calm him down/make him sleep on a long flight?

I have kind of a big problem coming up. I am going to fly back to America from China with my 3-year-old son, but my wife isn't going with us. We have already told our son about it and he seems fine with it now. But at the airport when we leave and he sees that his mother isn't going with him and me to America, he will obviously be extremely upset and probably become hysterical. My son and I are very close, but young children are usually closer to their mothers than their fathers. I worry about whether I'll be able to calm him down on the flight and even after we arrive in America. The flight is very long too. What pills, drinks, or whatever can I give to my son before or on the plane to calm him down? Thanks a lot for any help!