In the promo for 6x02 episode, you can see Sophie Miller's (Patrick's psychiatrist) face in the picture on the fridge. Above that picture is note from Red John. So, Red John probably found out Patrick's happy memory from Sophie Miller.
As for who RJ is, Partridge was my favourite, but since he is 99,99% dead, I will say McAllister only because we know very little about him.
