My son just started kindergarten and was in preschool last year. These are some of the things it would be helpful to him to know:
Recognize letters, capital and lower case, and the sounds they make
Recognize numbers ... maybe up to 20
Write letters and numbers as best he can (and this is tough... my son's motor skills kind of came together toward the end of this last preschool year so don't panic if this is a tough one for now!)
Writing their name ... capital 1st letter and then the rest lower case
I saw someone mention the Pledge of Allegiance and yes, that is something he learned in preschool.
Also, personal information such as birthday, address, phone number
Recognizing money (coins, what they are called, what they are worth, etc..)
They are working on all of these things in Kindergarten. Spending time in his class, I can tell you some kids know most of this, some are just learning a lot of it... so don't feel like your son has to be expert on all of this... just might help him get a jump start.
Read, read, read. It's good to run your finger under the words to he understands it runs from left to right, etc... My kids loved going to the library and picking out books. Story time at the library is another great thing... and free.
Try to make it fun. My kids always balked at sitting down for a "lesson". That said, there are preschool workbooks you can get just about anywhere that my kids found to be kind of fun at times. Oh... and cutting and pasting! Mastering scissors was something they worked on in preschool. Make letters out of clay, use magnets, puzzles...
One thing my kids liked is this go-fish game we got, and I've seen it lots of places... it has a capital letter and an animal that starts with that letter and the match has a lower case letter and the baby animal. They loved playing go-fish or using it as a matching game. Helped them recognize lower case, I think.
Just try to make things fun and look for teachable moments throughout your day... waiting in line, etc... you can ask him to read signs, count things, etc...