A.H.
Here are a couple of websites for you to take a look at that may help:
www.fromcouchto5k.com and www.coolrunning.com
Both give tips for beginners and a running plan to follow to help you get started. I used to run races all the time and I was not a runner. It is all in the breathing and pacing, so if you can find someone (perhaps your son's PE teacher) to run with you, even if it is just to help get you started is very helpful. They can show you how to stretch, breath and pace yourself. Running has to be one of the single best ways to get in shape and see results quickly. It is hard on the knees and feet though, so be sure to invest in the proper shoes and equipment to make the most of the sport. It is a lot of fun to run races. I did a bunch of 5ks (3.1 miles) around my city. After awhile you start seeing familiar faces and you always feel so great afterward. Often the races are held for a charity, like Race for the Cure, so that gives another level of satisfaction having participated for such a great cause. Good luck! What a great activity to share with your son!
HTH,
A.