re: his son's brain tumor, let's say this is true. Could it be explained by him noticing symptoms of it and subconsciously being suspicious, which led to the pink beam "telling" him?
I don't actually believe this, but it has occurred to me before. Here goes.
Phil (whose stories I love) claimed at one point that the FBI - or some outside agent - had broken into his apartment and detonated explosives in his filing cabinet. This could've very well been Dick himself.
Regarding his son, it was not a brain tumor, but a testicular hernia, or related, that his son had. Simply put, perhaps Dick was feeling up his son and decided to blame the finding on the light?
I believe what happened is that he likely found the hernia and then, in a fit of paranoia, decided to find another context for its discovery. You need to remember that Dick claimed to have seen a giant Teuton warrior in the clouds, looking down on him, as if a demiurge was making its presence known to him. This was likely a very paranoia-inducing event.
I am now reminded of the Anime Berserk.
Anyway, I do not believe that Dick was a child molester or false-flag operator against his own filing cabinet (although I am unsure about the cabinet one). We don't really know.
I think maybe he took a lot of amphetamines, and maybe he also had a mystical experience. Who knows? I have my own ideas but they may do more harm than good.
Here be dragons: https://www.youtube.com/watch?v=vXqHJYz8NXo
* This is your friendly reminder that human rights and a cessation of killing and torture globally serve as the bedrock for all modern law and that violent tendencies must be culled from our collective consciousness.