Both of my girls were under 40 lbs at 4 years (I remember because they have to be 40 lbs to get out of a 5 point car seat into a booster, and this normally happens at around age 4, but they were too small). They have to be 80 pounds to get out of a booster, and that didn't happen for older one until she was 10, and the little one was 9.
(if they reach the weight limit BEFORE the age limit, they still have to stay in the recommended restraint. Car seat is 4 years/40 lbs, booster is 8 years/80 lbs - just wanted to clarify, since the OP's son might reach 80 pounds before he's 8 years old. And he is NOT alone in that. )
Back to the OP, preschoolers often weigh about a pound for every inch tall they are. Your son should be about 43 pounds. Young children should not try to actually lose weight, but rather maintain their current weight until they "grow into it" as they get taller. Your best bet is to change the eating habits of your whole family, but as someone trying to do that very thing, it's MUCH easier to type it out than to make it happen in real life. Good luck!

You might want to start by keeping a food diary for your son for about a week, and see what he's really eating (before you try changing anything!) and how many calories he's taking in. Your ped or a pediatric nutritionist can look it over and give you some ideas of what to change, based on what he's currently eating.
