Hi K.,
When my oldest son, now 15, was in the 3rd grade his weight when through the roof, too. Three pant sizes up in one school year! It took my husband and I a while, but this is how we got it under control (and we're still working on it).
First, severely limit "electronic" time to a few hours on the weekends. For example, gameboys, Wii, Xbox, television, computer, cell phone games must all be limited to a few hours on the weekends. These things act like anchors and limit movement. (Yes, I know some Wii games are very physical, but you get the point.)
Next, encourage your youngster to play outside. Bike riding, scooter, trampoline, street football, basketball in the drive way are all fun things for little guys to do, if you feel your son will be safe enough playing outside. With the limited electronic time, "go out and play" will take on a whole new meaning.
After that, build physical movement into your routine. This summer, my son and I decided to start walking our dogs morning and evening. I needed to walk as part of a surgery recovery, the dogs needed walking, so we put it all together. Every day but Sunday, we're up and walking the dogs for a good 15-20 minutes. Is there a school nearby with a track? Try intervals...walk the straights sides, jog the curves. Increase the number of laps as you and your son feel comfortable. These morning and evening walks have turned in to great one-on-one time for my husband and I to spend with either of our sons and with each other...and we're all in motion together.
If outdoor safety is a concern for you, and you have the space, try a second hand sporting goods store for a treadmill (or a new one, if you can afford it).
Some elementary schools have set up voluntary jogging programs during recess to get kids up and moving. For example, my son's elementary school would give away little prizes (erasers, pencils, shoe charms, etc.) for kids who had run/walked "superlaps" around the school field. Perhaps you could start such a program at your son's school to encourage healthy movement.
Last but not least, rework his diet. After my son's weight ballooned, we stopped keeping foods like chips, cookies, sweets, even ice cream, in the house. If my sons wanted to snack, they could get fruits and veggies from the 'fridge. Baby carrots, apples, bananas, pears...these are all normal snacks in my house. Try to limit fast food, both at home and out, as well as soda pop. Last but not least, encourage your son to drink more water. We often mistake thirst for hunger.
Is my eldest perfectly skinny now? No, he is not. But as he puts on his height, we see him slimming down. These changes have been lifelong changes for our family. I know they work because my younger son avoided the weight gain because we had instituted these changes. My son will likely always struggle with his weight since our mutual favorite activities are reading and watching movies. But, we walk together, do our pushups and crunches together and I invovle him in physical activities whenever I can. He's not an athletic guy, so we make it happen at home.
Whatever you do, don't make him feel bad about his weight. Poor self-image is just as destructive to guys as it is for girls. Avoid comments like my mom said about my son, "I just don't understand why the girls flock to your son with that spare tire hanging around his middle!" I told her it was because he was charming and made them laugh. Her response, "Hmmmm...I just don't get it. It's not like he's buff or anything." Obviously, my mom and I disagree on what women find attractive in a man, but those sorts of comments really hurt my son.
Sorry this is so long. Good luck!