Invitations should be sent out at least three weeks before the party. Request an updated class list from your child's school; it will likely have the addresses and contact information for most of the kids he wants to invite.
Be creative with the invitations. They can set the tone for what's to come. One mother had a gardening party for her daughter's 6th birthday; she sent out invitations attached to seed packets, which guests were asked to bring with them to the party. The kids planted the seeds in a prepared garden plot (and were invited back later in the summer to harvest the fruits -- and vegetables -- of their labor). They also pressed a few seeds into small clay pots that they painted at the party and took home as favors, along with their own pairs of children's gardening gloves.
You can make invitations by hand or on a computer. Or have your child draw a picture you can use to decorate the invitation. Specify the day, location, drop-off and pickup times, and anything children should bring (such as a swimsuit or towel). It's also nice to note in the invitation what you'll be serving; that way, parents will know whether they should feed their children before the party and they can alert you to any food allergies their child has. Include the date by which to respond as well as your home phone number and your e-mail address. Ask any parents who won't be staying to provide you with emergency contact information for the day of the party.