CarolDW good for you in asking for advice to make changes.
First of all, don't go on and on to him about his weight. This has never made any fat person in the history of the world lose weight - quite the opposite.
But you do control what kind of food he has access to. Eg don't have pringles or chocolate in the house, even if you want them.
I'm a terrible cook, and I find it boring. I've solves this by making veg stew a lot - my DS likes this, it's amazingly easy to make, it costs very little, it's filling and comforting, and it gets a ton of veg down DS with no effort at all. We still have a takeaway from time to time (about once every 10 days?), but it isn't junk food every day.
At school, who pays for the extra meals and the cake and so on? If he is paying by meal, just give him enough money for five meals a week, then if he's eaten them all by Wednesday, a week or two of that will make him slow down on buying extra meals.
Mine has toast for breakfast every day, plus I start when he's still bleary-eyed by shoving a peeled banana in his hand, which he eats before he has realised it, thus filling him up a bit on a piece of fruit (and one of his five a day) before he eats anything else.
But the biggest thing, I think, which means although I am a fat parent with terrible food habits, but have a child who is pretty fit and has good food habits, is exercise. I don't drive, his primary school was quite a long way away, and so we had to walk. So, from the age of four he's done more than an hour's brisk walking every day, just to get to and from school. It's cheap, reliable and makes a huge difference to his fitness. (He doesn't have any other scheduled fitness activities, and is bored by most sports.)
Thinking of starting small, if he's a couch potato, does he have Wii sports?