@chappmom5 - sorry for the delay in my response. There is no harm in having your S send a letter to his admission counselor, especially if this will allow him to feel a little less nervous. While sending a letter saying he is interested may not have any effect on his decision (at this point), there is no downside to doing this If he does choose to write it, he should write to his admission counselor that is assigned to him.
Let me know if you have any additional questions - always happy to help!