Hi. I saw your post and had to respond as I was an overweight child myself. I agree with the ladies here, do not buy the junk food so he isn't tempted to eat it. Also getting him involved in an activity he likes I think is the key. Don't force him to do it otherwise its not going to work.
But let him experience different types of activities, there is rock climbing (a lot places have those indoor rock climbing things), hiking, canoeing, bike riding (this one was my fav. as a child), jump roping, karate (my husband took it as a child and he said he loved it and is very good exercise), skating, etc, etc. Just trying to think of things he might like to try.
Basically it boils down to just getting him excited about something that he is going to WANT to do himself. Because forcing a child is never the way to go. (not saying you have or that anybody suggested it) I am just letting you know what worked for me.
My grandpa watched me when I was a kid a lot and he would take me on long walks in his pastures (he was a farmer) and I absolutely loved that! So its like I said, get him into something he is going to enjoy.
Also allow him a treat every now and then as well...............
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