Sorry for the silence- Easter Day and at DM's with all the cousins so it's been hard to get a moment to read all these replies!
Thank you so much for all the encouraging comments and ideas as to how to broach the subject. I will try my best to answer some comments as I remember from my first read- through but when I get a moment longer will go through and check I have answered all comments
So genetics wise, DH is, as I mentioned, 6'6. He is a wide build too- very wide back and so very much in proportion. He has been a very successful athlete to high levels, especially in endurance type events and particularly in rowing which favours a tall, strong physique. DS hasn't got his DF's long lean legs but instead seems to have my legs. I have strong legs and a small rib cage, my DB is the same but is also tall and was a rower too. So as DS is not particularly tall as yet (not short but also not the tallest in his class) I imagine there is going to be a big growth spurt due- my DM says DB had one at about age 13.
When DS mentioned he was unhappy with his shape i didn't just ignore it, I encouraged him to get out running. I used to run and DS would do a park run with me but unfortunately due to a medical issue am unable to run at the moment so sadly we cannot do one together. However I am able to cycle, this is a great idea and I will do this. The swimming DS does is swim squad and school swimming and it works out that he is swimming 6 times a week and this one hour Lane swimming with squad and 30 mins with school. He comes out flushed so I know he is getting a work out. We also just got home from a week's skiiing and DS was pushing himself to the max doing black run moguls and off piste so he clearly has a level of fitness. Although his running is strange he does well in rugby as he is very strong and good at tackling but he obviously isn't the nippy one that gets the ball and runs for a try. Last term he did hockey which he is not so good at but he was generally in the second or third team of four. This term is cricket so lots of standing around, but also tennis and I think some athletics. Anyway, lots of sport at the prep school, generally including most of the day on Saturdays.
We have talked several times about ensuring he gets a healthy meal at school and he promised me that he gets lots of vegetables and chooses fruit at snack time. At home we eat healthily with lots of veg and he has the same size portion as me, not as big as DH. We do not have crisps in the house, I buy only weetabix or fruit and fibre as cereal and pudding is generally yoghurt or maybe an own-brand magnum ice cream as a treat. I know he is ravenously hungry as he always wants a bowl of fruit and fibre after swim training or late in the evening. I usually let him but sometimes I say no and let him have a glass of milk instead, depending on what he had for dinner.
I have told him that I believe he is going to have a growth spurt. It's encouraging to hear that this was the case with some PPs. I spoke very much along the lines that the PP's GP did but he pooh-pooed the idea. It won't be helping that he has spent the week with his cousins who are super skinny- my DB's partner is only 5' and has a tiny build whereas DS is like his dad.
I just googled what an overweight 11 year old should look like and he doesn't have moobs or anything but he does have a lot round his tummy (I probably exaggerated about rolls of flab but I think when he is sitting down it looks quite bad).
I gut feeling tells me that he's going to use the extra for a growth spurt but that as he is unhappy about it I am aware I probably need to have another discussion about food intake which is why I was wondering how to broach it. I want to discourage him from worrying about it. Anyone know of any famous sports people who were chunky as preteens? It would be helpful for him to realise that it is a thing.
Thanks again all for the replies, apologies again for the delay in relying