Nope. I don’t think it’s ever too early (or late) to start changing someone’s eating habits for the better. You might want to read the answer to Shelli’s question “Can I give your book to my kids?”
You might also want to consider that if you are concerned because your son has been in pretty good shape up until now but has been putting on weight suddenly (over the past 6 months to a year), you might not have anything to worry about.
Boys often put on a little fat at 10 or 11 – right before they go through the growth spurt that comes with puberty. A lot of that fat is going to be used for vertical growth and things should balance themselves back out once the spurt is over. I you look back at pictures of yourself in 5th or 6th grade, you’ll probably notice that you got a little pudgy too for a while.