Is a guy like this just too fragile? How can I help him?

If you HONESTLY found the letter by accident, then tell him you saw it. Tell him you are sorry what you stumbled upon, and tell him how sorry you are about his loss. BUT, if you found the letter by going through his personal belongings (hopefully this is not the case), dont tell him you saw the letter. Just tell him, "If you ever need anything to talk about, I want you to know that I am here to help you." But do NOT pressure him into telling you. Its a turn-off. Just let him know that you will help him with whatever is going on