The figure you quote of 85% or so of Americans being Christian is misleading--nearly all of these the nominal ("Christmas and Easter") Christians. The "evangelical" type is far more rare. George Barna, who specializes in surveys of religious life, suggests that about 7% of Americans have a Biblical faith.
This is important because this Biblical worldview does indeed believe that the nonbeliever is destined for hell.
I would agree with most of these posters about the rude behavior, but I condemn it not for the message but because it is an inept means of evangelizing Dad. Cut the kid a break though, most teenagers are frequently wrong but never in doubt.
Proving God exists (or not) is a complex issue, but as for the "scientific" proof He does not...well, it has problems of its own and most devout atheists refuse to admit to the problems.
For example: the Big Bang is now well established (scientifically)--but what existed before the Big Bang? In order to make the Bing Band work, physicists need to twist the laws of physics we observe into a unified force, which suddenly become nonunified--but have no explanation as to how these 2 conditions occur. In other words, we need to throw out everything we know about physics, BELIEVE that something occurs that we have no evidence for, in order to explain the world as we observe it.
And the Anthropic Principle: the laws of physics are so finely tuned in terms of physics to allow our present world that such conditions are extremely improbable.
And life arising spontaneously. There is a bigger jump in complexity from the non-living to the first bacteria than there is from bacteria to Homo sapiens. The evolutionists usually skip this one, or do "scientific handwaving" and claim it occurs, but don't have an explanation.
Now, nothing in this proves there is a God...but it does make the absence of one extremely improbable. But if your belief system is closed, if you only believe in natural causation, you have excluded anything Supernatural, then you won't believe in God. But you are starting with your conclusion, and then reasoning to it.