RJC Boy's Shirt Size Chart (Measurement of the shirt in inches). Always go for bigger size if you are not sure:
Size 8: Chest=32, Shoulder=15, Back Length=21
Size 10: Chest=33, Shoulder=15.5, Back Length=22
Size 12: Chest=34, Shoulder=16, Back Length=23
Size 14: Chest=36, Shoulder=16, Back Length=24
Size 16: Chest=38, Shoulder=16.6, Back Length=25
Size 18: Chest=40, Shoulder=17, Back Length=26