Red John is Bret Stiles' son, I'm absolutely convinced about that.

I don't know who that son could be, my favourite choices are Bob Kirkland and Ray Haffner (Bruno Heller: "Suspect list is legit"; both perfectly fit to Rosalind's and especially Jane's descriptions: "very ordinary looking man, mid thirties, medium height, thinning brown hair. He’s methodical and tidy, keep himself very clean. He has a small speech impediment that he’s deeply ashamed of. He works at a clerical job" - points more to Haffner with the clerical thing...). I have chosen Kirkland, cause I think he's the smarter guy: I wished RJ to be really smart and likeable, a totally winning character (which is why I would be disappointed with both Haffner & Krikland: I think, in that way, Stiles would be the best choice so far, although too old...)

Ok, let me know you that I'm from Germany, so I haven't seen the last couple of episodes of season 5 so far, but I know about the list of 7 suspects. Also, I don't know if someone else has already been typing on Red John being Stiles' son, if so, I share his idea.

My 3 reason for RJ being Stiles' son:
 
(1.) Red John has money and influence. Bret Stiles' son would definitely have money, and definitely get access to agencies, high society, etc.
(2.) Bret Stiles obviously knows Red John's identity, but neither is he getting killed by Red John nor seems Stiles to be one of RJ's friends, as he sometimes helps Jane and seems to be afraid of RJ. Stiles is like the father who doesn't want to betray his son though he knows that his son has chosen the wrong path and become someone evil. And: Both, Stiles and RJ have the strong talent to manipulate people... they are totally father & son...
(3.) Every writer needs his inspiration, even if it's a very good writer. I really think that there are some smaller connections between John Katzenbach's thriller "The Analyst" and the RJ story (do not missunderstand that: I absolutely don't want to say (!!!) the RJ-story could be inspired by "The Analyst", there are just some small similiarities): There is some kind of rebirth of the main protagonist as he sees no chance to win the game against his antagonist - Jane pretends to start a new life at the end of  season 4, Rikky (the main character of T.A.) pretends to kill himself; Red John and "Mr. R" (the bad guy in "The Analyst") both are extremely smart guys who seem to have an almost unlimited access to money institutions etc.; RJ and Mr R both regard the fight against Jane/Rikky as a big game; Mr. R. communicates to Rikky using poems (like "Tyger, Tyger"); the big debt Rikky carries with him is that he had not paid attention to people and had used them for getting a career - like the former "visionary"-Jane. And: Mr. R. has a very manipulative father who seems to give Rikky some hints to find Mr. R....