To me, there are a few major things that suggest that this journey is not truly taking place.
First, he seems too aware for it to be real. He is, for example, able to see the bullets that have hit the water and are drifting down towards where he is. That seems unrealistic.
Second, his consciousness seems sort of weird. He sees his own hands as separate from himself. He sees this vortex of color where everything (trees, sky, etc) is indistinct.
Finally, the soldiers' actions seem unlikely. This is especially true of the cannon. It seems so unlikely that they would fire a cannonball (not very accurate) at him. And then it seems unlikely that they would miss if they used grapeshot.