If the snake is small for a 2 year old, ignore its age and go by this rule - Give it a food item that is 1 1/2 times the girth of the snake. There really isn't any rule this age should eat this size because the snake may have been kept on a maintenance diet, it will catch up eventually but you will do more harm than good trying to rush its growth because you think it should already be a certain size. Without a picture and weight it is simply not possible to say what size it should be eating.
I have an 06 snake that you would think was 07 to look at... He was small when I got him. I'm letting him grow at his own rate.