He’s partially right, as the brain at least does need some amount of glucose. The part missing from that is that we can produce enough glucose to sustain the brain and any other process that needs it through gluconeogenesis as others have mentioned.
Part of the reason we are able to produce that much is because our usage actually decreases when there is not enough glucose available. The brain goes from running 100% on glucose to running about 30% I believe. If it continued to run on 100% glucose, he might be right that we can’t produce enough (though, i guess that’s uncertain, as it appears to be a demand driven process, so it’s hard to say what the maximum output is).
Regardless, if it was absolutely necessary to consume sugars to survive, there’d be a lot of dead people around the zero carb crowd, who get so little sugar it’s generally negligible (hence zero carbs). There have been experiments where people were only permitted meat mostly for a year while being closely monitored as well we could dig up (which also proved the body didn’t absolutely need vitamin C in all situations). That may not prove that no one requires some amount of sugars to survive, but it certainly proves that it is not true that everyone needs to consume sugars to survive, at least not from fruits and vegetables.
I wonder though, if he’s just like me in that his biology classes simply only when over the cell respiration cycle whereby glucose is turned into energy in human bodies. In mine at least I recall there being acknowledgement that other cycles exist, this was just the most common one. if that’s the case, he just needs to be shown what the other processes are (how fatty acids are turned into ketones and from there into energy, etc).