I would do some fun family activities to get active. For example, go for hikes, play baseball or soccer, bike together etc... You could also enroll the child in a sport or dance class that he/she would be interested in. I would also watch the food I was making and try to cut back on some calories, fat and sugar. Also try to get the child to drink more water and cut back on sugary drinks. Diluting juice with water is an easy way to cut back on sugar and calories. My kids don't even notice that I add water to their juice.

It would be ideal for the child to stop gaining weight, or slow down on weight gain, not actually lose weight, unless they are obese. If they are very overweight or obese then losing weight might be necessary. I would talk to your doctor.

Here are some articles with more ideas:

http://pediatrics.about.com/cs/obesity/l/aa012503a.htm
http://kidshealth.org/parent/general/body/overweight_obesity.html
http://www.myoverweightchild.com/

Most kids are energetic and playful enough that you shouldn't be needing to force them to "work out".

If the kid's being a sloth, then he just hasn't found an activity that's fun for him.

A common problem is if the kid isn't into team sports. In that case, take him swimming, or get a trampoline, or see if he gets into skating, or get him a bicycle...

The other thing is to just cut out the sugar.

The thing about sugar is that in and of itself, it doesn't really have a lot of calories, but it tends to come as an additive to oil starchy junk foods that *do* have a lot of empty calories, and sugar has a way of *tricking* your stomach into thinking you're hungry when you're not.

Specifically, if you eat a big breakfast without sugar, like bacon and eggs, even though there's a lot of calories there, your stomach won't feel hungry until you really are, but if there is sugar added to that same mix, say... a syrup on some pancakes, then what happens is that after about two hours the digestive processing of sugar causes a slight buildup of peroxide free radicals in the stomach which creates a subjective sense of feeling hungry when actually your body is not in need of nourishment at all - it's a false signal - such that you'll start looking for donuts or something like that... notice how that corresponds to morning coffee break time.

The whole problem with people in north America being overweight started to snowball after 1973 when a chemist discovered a cheap way to convert corn cellulose into liquid glucose, such that it suddenly became very very cheap for junk food processors to add lots of liquid glucose to everything, and glucose, after two hours, will trick your stomach into thinking you're hungry when actually you're not, and so people would start gorging more and more on things like Twinkies.

That's why the Atkins diet works. It's got lots of calories.... after all, it's all meat and fat, but because there's no sugar, your stomach never gets tricked into feeling hungry when actually you're not. (It also means that an Atkins diet will still work if you allow bread and potatoes to be part of it... just keep out the *sugar*).

So... if the kid won't exercise because eh hates sports, well, maybe it's because all the other kids on the teams are goons, so help him find a solitary sport like swimming or skating or skiing where he doesn't have to deal with grunts, and if he's munching a lot on sweat stuff, replace his snacks with non-sweet things like chips and peanuts, and even though those have just as many calories, you'll probably see that he won't much on it as much as if it's sweet stuff.

New studies show that exercise does not have the large impact on weight that was previously thought. Therefore, I would suggest focusing on exercise last. Additionally, being heavy makes children sluggish.

First you should make sure that the child eats a healthy-sized breakfast, a normal lunch, and a good dinner.

Then, make sure the child has no snacks in between meals and no evening snacks.

Finally, make sure the child is getting plenty of sleep. Not enough sleep tends to make people eat more to compensate.

Honestly, I think the main reason why children today are so overweight is because a lot of the food we eat is overprocessed. Avoid things like corn syrup, refined foods, and PLEASE avoid margarine like the plague. Switching to butter made a huge difference for me. The more fat (via dairy) you eat, the easier you lose weight. I have no idea why but that's just what happens. Start paying attention to the ingredients in the food you eat and choose quality over quantity. The less processed the food is, the fuller you feel eating it.
Exercise is a little overrated. I've walked at a regular pace for 15 minutes 2 days out of the week and seen dramatic results, more dramatic results than when I was doing sports 3 hours a day 5 days out of the week. A child shouldn't be sitting down all day, but they don't necessarily need to run laps.