Thanks for the replies.
He usually gets one Weetabix in the morning with full fat milk. He had the option to have more but he chooses not to. He usually opts for a glass of water at breakfast too. Sometimes he will have porridge or a boiled egg with toast at the weekend.
He gets a small Tupperware of cucumber, sometimes baby tomatoes and a tangerine/apple/banana for snack at school. He has school lunches - god knows what they are, he can never remember to tell me. When he does it things like beans and toast. He did get healthier packed lunches before but he wasn't eating them as just wanted to go and play. He also takes a bottle of water to school.
He goes to the childminders after school and will have a snack or fruit there. She is a great childminder and I trust she's not filling him up with crap. Usually she will say if they have had a special treat.
Dinners for the next few nights are spaghetti and meatballs cooked in pasatta with basil and oregano (usually 6 meatballs) with some salad, homemade lentil soup with two slices of brown bread, salmon with mashed potatoes and carrots and broccoli. Lunch at weekends with me would be sandwiches with fruit, tomato, cucumber or carrot sticks and maybe a yoghurt.
He has been going to swimming lessons for around a year on a Saturday morning and loves swimming. I absolutely hate it but will start trying to take him before/after his lessons or on a Friday evening. He has just started playing basketball on a Tuesday evening (this week coming will be his third week). He loves it, it's two hours long and he comes out dripping in sweat so hopefully that will help. Gym in school is an hour twice a week. He goes to the park with the childminder after school regularly (weather dependent). We usually go a dog walk twice together at the weekend and DS usually goes on his scooter. The walk is around a mile round the loch then he plays in the park for an hour or so but I can probably quite easily make it a bigger walk.
He loves trampolining and there's a really good place near us so will start trying to go there every few weeks. Hopefully if I increase our walks and swimming it will make a difference. It's quite difficult to do things in the evening on a school night as we don't get home to about 6ish but could swap movie night on a Friday to swimming night!
I did send his dad a general message about it just saying we should probably keep an eye on it but I just have a feeling it's not going to go down very well. I could be wrong though - hopefully I am!
I definitely won't say anything to DS, I'll just try and tweak a few things, up his exercise and try and make sure everyone is on board from his dad's side of the family! Thanks a lot for all the advice.