It is obviously lens flare and there is a ray of light that points exactly onto the dog. So much is established, but why do these facts exclude the notion that the timing and the coincidence of these facts [asking if he is there, light pointing at dog etc] are due to the son's wishes to communicate?

Believing in an an afterlife does not always have be spiritual, as spirituality is only as of yet unproven science. Quantum physics for example can be used to explain apparitions, premonitions, telepathy and more on purely scientific grounds. Therefore it is not entirely impossible that our reality [i.e what we perceive] can be bent. Either by influencing our own senses or by using external easily manipulated energy, such as photons.

So it is not entirely impossible to think that someone in a different reality [dimension] may be able to do so. And because it is scientifically explainable it is entirely possible. The only thing that seems missing, is hard evidence but the amount of circumstantial evidence makes more than up for it. We should not underestimate people's intuitions [or gut feelings, which have been explained by neuroscience] as they date back to a time in which our basic brains were more in tune with nature [or reality/quantum world].

In order to believe what I just wrote, you absolutely have to believe in some sort of afterlife [which ultimately should be scientifically explained]. If you don't, the story stops at lens flare and the coincidence. It also shows that you should read more.