One game my DD invented once she knew the letter sounds (mostly from playing on Starfall.com) was that she would ask me to give her a word, and she would tell me what letter it started with. She would do this for as long as I could stand it! LOL.
I believe that CM included learning letter sounds in the ‘informal learning’ category. In other words, that would be before age 6. Her “formal lessons” would be actually putting sounds together and learning words. So you can start there.
There are lots of pre-reading skills, such as being able to identify rhyming words. Here is a list I found that might give you some ideas.
http://www.familylearning.org.uk/pre-reading_skills.html
All that being said, I couldn’t have prevented my DD from learning to read before she was 4. She did it herself. Then when DS hit 4, he insisted on learning, because his big Sister could read when she was 4, so he was determined as well. I wonder if there would be advantages to having waited, but this is how it turned out for us. My youngest just turned 3, and is a very different personality and interested in different things, so she may not read for a long time. I intend to follow her lead as well.
I think the key is not to start formal lessons just because YOU want to. Forcing the issue before the child is ready is almost always counter-productive. If your son loves playing with words and sounds, go with it. Otherwise, don’t worry about it.
Joanne