What is low lung volume on babies mean? My son is a year old and had a chest X-ray done and this came back on the paper they gave me.

Usually . Usually this refers to the fact that infants cannot voluntarily take a deep breath and hold it like adults are instructed to do when they have cxrs taken. This is often reported and may reflect that your son was crying at the time the image was taken. Recommend discussing with your pediatrician if your son has any acute symptoms (cough or wheezing).
Ignore it. This is just the radiologist explaining the limitations of the study.When a baby/kid/adult does not take a deep breath for a chest x-ray, the study reflects a lower lung volume and is harder to interpret for subtle changes. The standards for defining a totally normal study rely on a fully inflated lung.