School dinners.
If not an option, then carry his packed lunch in the front seat and don't let him hold it. give it to the teacher who is not to let him have it until lunchtime. He should not be allowed access to it at break and the school should back you up in this.
Until they are old enough to take responsibility for themselves, then we as their parents have to take any and all practical steps to physically stop them, if we have to.
I spoke to my kids school and gave strict instructions on what they could have and that included only being allowed a half portion of pudding if it was anything other than fruit. [meanie] I signed them up for all the clubs that are physically active - cross country running, athletics, etc.
You have to act in their long term best interests and sometimes that means taking total control in the short term.
All this happening at the same time as talking it through again and again of course.
You see, a fat child will usually become a fat adult. I cannot control what they eat when they are grown up, but I CAN control everything at this stage, and give them the most healthy start, get healthy eating and exercise habits started now, and hope that they are totally brainwashed by adulthood.
Ds1 was chubby at one point because he just loved his grub . He is not now, he's like a whippet! ds2 is chubby at the moment and I am working on it. This is totally my fault. ds2 has anaemia and was skeletal, really, he never ate and he looked like a famine victim. At one point we were force feeding him complan , he has been on medication for 2 years now and at first we were so happy that he was eating, that we let him have what he wanted, now he's gone too far the other way and we have to pull him back.
You can do it, you just need to be hard and focussed and to think what is best in the long term, what you want for him as an adult.