A letter is not required, but if it makes you feel more confident to have one and you are going to the doctor anyway, go ahead.
The letter should talk about the needs that your DS has, not his diagnosis.
It sounds like you would be making a special trip for the letter (if I'm reading your post correctly). I wouldn't do that.
Since you have the old GAC, I would just take that. Show it to the CM and explain that you got it on your last trip. If it met your DS's needs, you can say that it was helpful in meeting his needs. If it didn't work so well, explain how worked well and what needs he had that it didn't help with.