Once you get the go ahead for your son to sit the 12+ they will send you a pathetic pack of sample questions and an answer sheet for you to play around with.
However, I will see if I kept ours from last year, so you will be re-assured that it is no big deal...
The questions types, 85 of them in 45 mins, are of the same standard as the 11+ just a few more in a bit less time and the only comment my son made was that rather than have an example of how to work out the next section of questions, they sometimes had a wordy explanation, which was fine and very straight forward.
I found my son's knowledge in general had moved along big time since the 11+ so he found the NFER papers easy, Susan Daughtry good, the new short papers from The Tutors on this site would be worth purchasing.
Hopefully, as you were a near miss before, your son will have remembered the basic techniques and you will just be refreshing him on the various types - we tended to talk through the question types more and looked together about any he got wrong in practice, and just made it all quite laid back.
I will pm you once I have found our pack from last year, but once you get back into 11+ mode
I hope you will be pleasantly surprised at how much more mature they are