I think your doctor's concern is that the growth pattern has changed, more than that your child is small for his age. Your child going from 35th percentile to 5th percentile is the cause for concern, not that your child is simply small. With any kind of serious recomendation of this sort, I would get a second opinion. I would also expect the doctor to run hormone level before prescribing anything. You might want to consult a pediatric endocrinologist for this. I would not expect any doctor to put such a young child on this sort of therapy without a broader set of indications than one exam where the child is smaller than expected.