Preschool is a big step, and it's great that you're taking the initiative to prepare your child for some of the ideas and skills that he's going to encounter. Here's an article that discusses the sense of independence and social ability that will be helpful for your child to be familiar with when he starts preschool: http://www.education.com/magazine/article/gearing-preschool-development/. It even has ideas for how to assist your child in developing these important skills.
It also may be a good idea to practice counting and looking at letters--these are some of the earliest academic ideas introduced in preschool.
And perhaps one of the greatest ways to prepare him for preschool is to help him develop a love of books! Here are our picks for the best summer reading books for kids entering preschool. Try visiting the library, checking a few out, and then reading together before bed each night.