If he gets used to "liquid lunch" he's going to develop a taste for it. Meal replacements like Boost or "instant breakfast" drinks or even (to an extent) drinkable yogurts will fill him up for sure -- but he will never really learn how to time his eating and get his real foods eaten in the time allotted. I agree with the person who said to visit him at lunchtime. Watch for these things to get to the real root of the problem:
How long do they have for lunch, really? Does he have to wait to eat while other kids, who buy school lunch, get their food, or can he go ahead, sit down and begin eating while they are still in the cafeteria line? (Assuming here he's in school, not a preschool or day care.) He and other lunch-bringers should be allowed to get started while other kids are in line. If he purchases "school milk" to go with his lunch, stop! That really slowed down my child's kindergarten lunches because she had to wait in the loooong lunch line with the lunch kids just to get her carton of milk, and that wait often consumed much of her short lunch period! I quickly learned not to have her buy anything in the lunch line, even milk, and sent everything she would eat or drink.
Is he so busy being social, or just being distracted by other kids and noise, that he is unable to focus on food? Cafeterias are hugely, screechingly, horridly noisy most of the time and that would put anyone off food. And if he's young, he may still be so excited to be at lunch and off the leash for a few minutes that he may simply not focus at all on eating. That's typical.
Does he possibly have trouble with the foods being sent - For instance, do some foods you pack require a spoon, and you figure he'll get one in the lunchroom? He may not be able to get one soon enough or at all. This came up at my child's old school; sometimes she could not get the attention of the adult "table monitor" for the entire meal because it was so crowded, and kids were not allowed to get up and fetch their own spoons etc, so often "spoon food" that she had opened got pitched out uneaten when she ran out of time and never got a spoon!
These are the kinds of things that parents may never know are going on unless they visit the cafeteria during lunch and really observe things. The school may also have overwhelmed adults who are in charge and might need more adult volunteers during lunch hours -- something you can consider doing.
But back to the drinks -- seriously, if he has the equivalent of a milkshake (Boost) or smoothie (even a healthy one) daily, he's going to develop that sweet tooth, and getting him to eat balanced foods at lunch will be more of a struggle, not less. Go for finger foods he can open totally on his own and eat without utensils, and don't over-pack for lunch. Do go and observe.
Does his class have a snack time? Does he eat then? If not -- talk to the teacher. Even in sixth grade my child's class has snacks outside lunchtime.