Bilirubin is a normal chemical found in everyone, and is a byproduct -part of the normal mechanism of breakdown and recycling of the components within red blood cells. It has a yellowish colouration, and is produced in the liver.
Normally, levels of bilirubin withn adults is low, so I think what your grandson has is elevated levels of bilirubin - its not a disease in its own right, more an indicator that there is something out of balance within the body.
Because of its site of production in the liver, and because of its link to the the recycling of red blood cells, an elevated bilirubin level is usually considered a marker for problems effecting the liver or the red blood cells.
The cause can be wide-ranging; a reaction to certain types of antibiotics, through to hepatitis (inflammation of the liver) due to viral or bacterial infection. It could be down to a blockage of the bile duct, or gallstones, or it could be something that it causing increased haemolysis - some drug therapies, for example, or a disease like sickle cell anaemia.
Difficult to offer any other information or advice without more information, deanna.
http://www.labtestsonline.org.uk/understanding/analytes/bilirubin/tab/test#when