Advice re son not eating any lunch

Hello everyone. As I've mentioned before my 4 year old boy with autism is a fussy eater. Each day I ask him what he wants for lunch ( for school) and I put that in his lunch box. Often when I've picked him up from school I check to see what he's eaten and I'm lucky if he's eaten one thing. Usually a yoghurt. It really worries. I can't believe the dinner ladies allow him to go out to play when he's hardly touched his lunch. I have mentioned it to his teacher. He said they have tried just letting him have some of his drink rather than all of it, otherwise he's full up on just juice, but it doesn't seem to be working.

Please can anyone give me some advice. Maybe your child is the same?

Thank you for reading and I hope someone can offer me some advice.