Buying does not guarantee your slow eater will even have time to eat! The kids unfortunately take a while to "get the tray" in one piece back to their seat and then goodness if something is not just right or has to be cut up, they have to wait for someone. I'd personally plan on your childs favorites in small pieces to ensure good quality and enough to sustain him.
My son takes lunch, we include turkey roll ups, organic petite carrots and bottled water, usually pretzels and a cookie/2. Had something in the bag as a snack to hold him over.
They won't "give" the kids water at school (as part of the bought lunch) but they'll give him Milk, just Kills me, they say water has no nutrient value, well I don't know about anyone else, but MILK is not all its cracked up to be. Besides the child is allergic to it, don't get me started......