My son is 6, 3ft11 and weighs just under 3 stone. He is very active though, never stops running.
On an average day he eats (example):
Breakfast: Weetabix, banana
Snack: apple, breadsticks
Lunch: Sandwich with quorn ham and cheese, or with hummus, Vegetable sticks, crisps, yogurt, fruit
Snack: Fruit, or one plain biscuit, or cereal bar
Dinner: Lasagne or spaghetti bolognese made with quorn, or vegetable risotto, or quorn fillets with potatoes and broccolli, followed by yogurt and fruit
Bedtime: Milky drink.
It does sound like your son is overweight. But personally I would try to make small changes before making a big issue of this to your son, taking him to the gp etc. I became a chubby child around the age of 9 or 10, and later developed an Eating Diisorder for which I was hospitalised. I'm not saying that to make you panic, but to let you know you're doing the right thing in not burying your head in the sand and in taking some action. Your son is young, and his relationship with food can be changed now, which will benefit the rest of his life. You're a great parent doing a great thing.
Just focus on maintaining a positive and healthy self image, don't link weigth with self worth, never make him feel greedy, and don't single him out, I would talk about eating more healthily as a family without mentioning his weight. Talk about how our bodies need all kinds of foods, some they need more of than others, etc, then focus on healthy balance, and becoming more active - and your son's weight will hopefully stabilise and he will slim down as he grows. If you don't see that happening then maybe consider the gp at that point?