6 Year old son has gained almost 16 lbs. in one year! HELP Me

My son just turned 6 in March - He weighed in at 81 lbs. at the Dr.'s yesterday, I was shocked because he was 79 two weeks ago, and then prior to that about 3 weeks, he was 75 and one year before that he was about 65 - what is going on?  He is not sedentary, plays outside, inside goes to school, has gym.  He eats pretty normal I would say, althought when I first realized he had put on 11 lbs. in one year, I cut out school lunches and started packing his lunches, has not helped.  His lunches include for example a fruit cup, a GO GURT, a 1/2 peanut butter and jelly sandwich and he has milk with it at school.  I rotate that with main dish as spaghettios or chicken noodle soup.  90% of the time he does not eat all of it. We have normal dinners - I'd say we eat out or order in about 2-3x per month (pizza or Mcdonald's), but he has chix nuggets.  Yes, he does have major cravings for carbs like french fries, corn dogs and sweets, but I limit them to 1-2x per month if I can.  He's just blossoming a little too much.  He's clearly overweight.  He drinks sugar free Koolaid (no calories, no nothing in it), drinks maybe 1 diet pepsi a week, otherwise, drinks water and skim milk.  

What medical problems can cause such rapid weight gain like this?   I realize there may be some issues with the way he eats and needs more exercise, I just think this rapid of a weight gain there has to be something wrong internally.  Maybe a low thyroid problem?  

He has a low tolerance for exercise as I do. I was just diagnosed with Fibromyalgia (I am 38). I bought an exercise DVD, actually two of them and we tried it together and he pooped out on me not even halfway through.  

Any suggestions? I can take constructive criticism ;)

Thanks.

[Renee Radenberg]

I suggest you have your son get checked out by his MD and have blood work done to see if his thyroid is acting up or not. It is recommended to increase children's physical activities to loss weight. Have your child go and play outdoors, run, walk, and dance. Limit time in front of the TV or non-active-video games. There have been studies showing the longer the length of time a child sit in front of the TV, the greater the weight gain. I suggest you do limit high sugar drinks/snacks and replace with low-fat milk/yogurt or fresh fruit/air-popped popcorn/low-fat cheese. High sugar drink and snacks do not give the child nutrients they need. Eat low fat meats (chicken, fish, loin of pork) or non-meat sources (soy products, nuts), low fat vegetarian combinations (rice & beans, pasta & beans, low fat cheese & pasta), substitute dairy products (low fat soy milk/soy yogurt), use plant oils, non trans-fat margarines, and eat a lot of fruits and vegetables. Eat daily high fiber breads and cereals (oatmeal). Thank you for your question.

[spankyandrex]

I should also mention, he does have some learning disabilities - he's been in Early Childhood since 3 year old. He has verbal apraxia, used to have Sensory Integration Dysfunction and he will be going through testing/eval to seee if he's somewhere on the "spectrum" in June.  I don't know if this is helpful or not..........his language has flourished in the past year, .....along with his weight ;)