I'm worried about my 10 year olds eating habits

[Monkeypopcorn]

DS is 10. Is a confident, funny, intelligent boy. Has always enjoyed eating a variety of food, lots of veg and fruits. Recently he is constantly saying he's full, doesn't eat breakfast anymore because he's not hungry, has the same chicken wrap from the lunch hall each day and will eat half his dinner and say he's full.
He's always been very good at saying no to sweets and treats when he's full, can easily be given a bag of chocolates and then put them back in the cupboard after eating 5 prices, so that isn't unusual but when we go to the shop he is saying no to having treats more often.
He is slim, no parent rose coloured goggles on, he is definitely what you would describe as skinny. However, he has started to mention he thinks he's fat, it doesn't come up this often but occasionally after I say you're so handsome or something similar he'll snap back no I'm not I'm fat.
He has started going to the gym with his dad, we are separated, he sees his dad sporadically and clearly when he does have him can't miss out on a work out so takes him with him. Dad is very fit, muscular, used to be a body builder, eats tonnes of protein. I am about the right weight, occasionally creep closer to top end of expected weight for my height after holidays/Christmas but eat well the majority of the time. I only walk and swim for exercise so am just sort of average, no muscles.
There's no way the amount of food he's eating is enough for him, especially as he's only half a foot smaller than me at the minute and growing. I just don't know how to handle it. If I take him to the GP does it make a big thing out of it and possibly make it worse? I just don't think I know the right way to deal with it on my own.

That must be so worrying @Monkeypopcorn and I haven't really got any experience.

I would though have a look at this guide from the Caroline Walker Trust. It will give you an idea of how many calories he needs and how to achieve that in a good way. I didn't have a problem with my DS under eating but he was making very poor choices so I read the guide and got him to sit with me when he was open to talking about it and showing him some bits of the guide and how he could eat better. He took it on board, thankfully.

Another way is to perhaps show him how much calcium he needs in a day to grow like his DF and ask him if he feels he gets enough from what he's eating and how he can get more.

If you do think though that this is the start of an ED there is a dedicated ED Section on MN. I'd also advise talking to BEAT and his GP

Thank-you so much for your reply.
Going through that guide sounds like an excellent idea, we will do it as a family so he doesn't feel it's directed a him. Hopefully that stops it in it's tracks!
I'm just worried that he's obviously thinking about it while making choices. So worrying as I know I went through similar when I was about 14, was never picked up on because I don't think it got too bad. But I was stick thin, look back at pictures now and remember thinking I looked fat and now I can see I was desperately under weight!

You should have a word with his dad about shy conversations he might be having with him about fat and muscle. He may be trying to emulate his dad but cutting out fat - but not quite mature enough to realise what he's doing.