I agree with the "don't push it down his throat" 100%, but I also think encouraging kids to exercise is terrific, and I don't think 7 is too young. No, you shouldn't try to get him to jog 3 times a week, but you can encourage him when he does run (even if in playing or whatever). I think most important is being a role model. Since I run all the time, my 3 and 5 year old boys constantly want to go with me. We run the cul de sac together (about .2 miles) once every couple of weeks. These days my 5 year old likes to ride his bike alongside DH when he runs (3-3.5 miles).
After my first marathon, I went to cheer my cousin on in hers and I asked my 5 yo son if he wanted to do the kid's race (.6 miles). He did it, he loved it and he is so proud of his medal (they all got one), t-shirt and bib number. He also learned that you do your best by running your best - i.e. you don't have to win the race. Both boys did a kid's dash a couple weeks ago when I did my 1 mile race and they loved it.
So, my advice is give DS the opportunity and be a good role model and he may just turn into a runner (or anything other than a couch potato!).