Well, by flashes I mean memories of porn I had seen, as well as general sexual thoughts and urges. The strength of these vary, from simple memories that I can simply push away in an instant with ease, to thoughts and urges that persist and make me feel uncomfortable. These flashes come out of nowhere. You could be thinking of something and then all of a sudden your mind just takes a sharp turn into porn territory for no reason.
I do tell my wife whenever I have these flashes. I feel being honest with her is crucial, since she's helping me fight my addiction, and it goes a long way to repair the damage I have caused all these years.
I can't really say if other people have these flashes or if it's just me. If it is an addiction, I would imagine that he does experience them, but is probably too scared to say something. He could possibly not want to appear weak, but these flashes come against our wills. It's not something we actively conjure up, they just appear out of the blue.
Just let him know that you're there to support him, and keep him strong to prevent him from relapsing. I don't know how long these flashes will last, but he (and myself) has to hang in there. It takes time and strength to properly get over an addiction, and he needs all the support he can get.