J.:
You are doing the right thing by being concerned so that says a lot, also keep in mind that you have a family to protect and no one will take care of you and your son/daughter as well as you. The question around HIV shouldn't be relinquished to people you suspect of, you should ask this of anybody you plan to be intimate with b/c your health should come first.
If you are truly interested in this person for a long term relationship then feel free to talk about it, no letter, this is not only for him but for you as well. For example if you two are planning to take the relationship to the next level then have the conversation and both of you should get tested and emphasize that this is not about him is about the two of you. He should be aware of your health as you should be aware of his. You guys seem to have been friends for a while so I am positive he will understand.
Again no one cares for you as well as you do so whether is him or anybody else in your life you plan to be with, the HIV conversation is one you should always have it is your life we are talking about here and nothing is worth sacrificing it.
If your only interest in him is just being friends then be a good friend and let it go and he will tell you when he feels is right. Rumors are always there for a reason but they are rumors. If it is true this is private and it may be something he is not comfortable sharing with you yet. Just educate yourself about the disease and learn what kind of precautions you should take. After all it has been established that casual contact is not a way of contracting the disease.
Wishing you strength and success.