A way to monitor food intake is to check your son's food container. However, he may very well give his food away!
You can also befriend the teacher so it may be easy to persuade her to take photos of your child eating. It may be a bit extreme, but if it gives you peace of mind, then go ahead and do it.
Another thing you can do is to exert the effort to prepare food that your child absolutely loves so there's more of a chance he finishes everything you pack for him.
However, in the end, if you are truly worried about his appetite, then head to your doctor to discuss normal weight for his age and if you need to have him take appetite stimulants.