Best thing for a child that age is to keep it simple. Definitely cut out the junk food, and if possible cut out processed food altogether. Swap white bread/pasta/rice for wholemeal, and cut out sugar wherever possible.
Try to make sure your son is getting a serving of lean protein (chicken, lean red meat, fish, eggs), plenty of veg and some healthy carb (as above) at every meal - you may also find it helps to give him four small meals every day rather than three larger ones.
The best thing you can do to help your son lose weight though is make sure he's getting plenty of exercise every day; kids these days get nowhere near enough. Not only will that help to keep his weight in check, it will also build his muscles, bones, and keep his heart healthy - all of which will set him up for better health in adulthood.
Best of luck!
Young children shouldn't diet. However healthy food habits should be formed at a young age. Cut out all junk food and fast food. Provide plenty of fresh fruit for snacking. Also stock up on natural, non-processed, cheeses, whole wheat crackers and breads. Try to buy as many organic and non processed foods as possible. Experiment with all types of veggies, and continue to offer them even when at first rejected. Have him take at least one bite of everything. Various studies show that after a certain number (usually about 13-17) of exposures to any food the palette will develop a taste for it. This is especially true for children. Nuts also make a good snack in limited qualities. Also organic, low fat, reduced sugar, yogurt is great (no artificial sweeteners). For drinking cut out most fruit juices, soda and kool aids. Instead provide water and milk. The occasional glass of natural 100% orange juice, or organic 100% apple juice (like 6 oz a day at most) is okay but should be offered very sparingly. Exercise is also very important, turn off the tv and get rid of the video games. Provide a bike, bubbles, soccer ball, basketball, whiffle bat & ball, and whatever other outdoor toys you can find. For indoors you can get a small trampoline with a handle bar, an indoor play gym, a sit and spin, etc...
Doing these things will help your child to grow up active, healthy, happy and well adjusted.
Just throw out all the junk food, he is young, if you wait longer to do this he wont let you he will be addicted.
My kids have Md's on the first Friday of the month and they like it/look forward to it! and they know this. You are the parent and at age 4 you have the chance to be responsible for what he eats. If you leave it any longer, habit will have set in.
Make food fun ! which it is, get him to help you prepare some food. What does he like to eat ?
There are healthy alternatives to most things .
Eg. Ice Cream. If you skin and slice a banana, freeze it, put in a drop of pure vanilla essence, then blend it, you have ice cream ! But it is just banana, Try tricks like this and say NO, say "I am your parent and i am not letting you eat that" Health is the best thing you can give your kids.