Drying Up Milk Supply

I've been EP since day one. I'm going to leave for BMT for the Air Force during the summer and my little one will be 7 months old. So I've been pumping for 6 months. He'll have to switch to formula when I leave. Any tips on how to make myself stop producing milk? I produce a lot. He usually drinks 7-8 ounces every 2-3 hours and he sleeps through the night. Also, any tips on feeling guilty on switching him from breast fo formula?