Does your son eat lunch at school or take one from home? Same with his breakfast? I have heard of so many kids who eat a healthy breakfast and dinner at home, but have fries with gravy and a huge burger, plus a pop with chips or chocolate bar for lunch which is equal to or over their entire days needed calories in one meal. We had friends that had two very overweight children who they bought large size bags of chips, nachos, and candies for every day and the mother said they were watching them.

One day she bought them each a large bag of chips plus a chocolate bar & a pop for a snack (well over a thousand calories). I'm talking about a 270 grams + bag each; and ... sometimes, grammy and grampy were doing the same thing while she was at work; and that was just for a snack, not a meal.
So ... like JAYELL says, you really have to watch what they are eating at school, at friends, at relatives homes, and anywhere outside the home without coming off like a sergeant major. A doctor once told me (when I was young) that children and teens don't need to go on a diet, but to just eat 3 balanced meals with 2 healthy & portioned snacks a day; and that eventually their weight would balance out as they grow. He said it was unhealthy snacks and food choices that caused the overall weight gain over time, and I know in my family that was certainly the truth (but weight issues ran in the family on both sides, so we needed to watch even more).
He had me write down everything we ate for two weeks and noticed that we weren't eating enough veggies and fruit, so he said he wanted to see two veggies on the dinner table every day plus 1-2 fruits each day for us kids. He said it was fine to have one serving of a treat dessert on Sundays, and the rest of the time to have things fruit, yogurt, or sugar-free jello or pudding.
It may be due to his body type and age and height (?), if you have ruled out all the things we mentioned above. I know a boy that looked chubby from 10-13 (and was eating healthy), but had a growth spurt from 14-16, and is now almost 6 feet tall and slim; it was his body type but that changed at puberty. Sometimes, between 7-12 children appear chubby, then they spurt up at puberty (which is different for each child and younger today than ever before).
And since you just started your family on a new healthy eating style in December, I would just give him more time. Have you thought of journalizing your daily foods for two weeks; that's what my doctor did for me, so he could see where we needed to make improvements. I would make this volunteer and like a game for your son that you could do together; maybe ask him to help YOU, so the onus isn't on him and there's no pressure.
You could go see a dietician for advice as well; they are very helpful and real pros in this area and can offer lots of tips and advice. As kids, we were very active year-round: outside running around every day from the time we got home from school and after dinner til bedtime and on the weekend in sports and outside games as well like baseball & biking, skipping & soccer, skating plus road & ice hockey. Your son is fairly active in the summer, but what about the winter? He needs DAILY activities all year round ...
PS ~ I was typing while you responded? Yah, lunches at school are a big issue today, and hard to monitor. We didn't have that issue as children. I might eventually try for packed lunches all but one special day, but would that upset him too much??? Maybe every other day? Oh, I see he's game for this ... so go with him on it and have one special day, say Fridays for fun. Yah, have fish, chicken, and turkey some dinners as well if you can. I think you just need more time to iron all these things out ... good luck!