I really can relate to how you feel. But I think you have been already brave to tell his son the truth. Telling your T will surely be more difficult but I do think it is better when you tell him instead of he maybe figuring it out on his own. I am sure his son will tell him and maybe your T can make the link after all.
If you are planning on giving him things that you have written to him anyway why don't you write this up as well and give him and tell him that the only way to let him know at the moment is through writing because it is too difficult to say for you.
I am sure he will be able to understand and appreciate you being open with him.