I have three, count em three deaf sisters. My youngest started public school at 2 and 1/2 to get a jump on things, and boy did she ever! She was only behind in speech all through school, she even tests at Genius level on IQ tests.
My mom and dad never learned how to sign. I was the only sister who could hear, and after countless classes I can't sign very well (though I am better than my parents) and the simple answer to why is guilt.
It is a very positive step that you want to learn sign language. You have to be really pushy and fight hard for your son. You need IEP meetings when he goes to school. It is not about "labeling" him, but rather that you are identifying a need that must be addressed in a particular way. My sisters are not "disabled" at all! One needs to write things out for hearing people because she is completely deaf and her speech is just not clear. Another needs phone assistance (in fact we laugh because she babysat us as kids, and being the only one who could hear, she would "Demand" I call my mom at work to tattle on myself- yeah right!), and the other just misses a few things.
The key is to treat him normally. We had relatives that would yell at all of them in an effort to be sure they could hear and even when they couldn't they took it as the relative caring enough to try and treated them respectfully. As a result of growing up with them, even my voice is naturally elevated now, and I ennunciate clearly. My kids have (finally) figured out that you have to repeat yourself and speak loudly and clearly with their favorite aunt. They are learning sign, but don't really have to use it with this aunt (the only one who lives close) so she lipreads and can hear some.
They say my parents would have had no deaf children if they had married other people, they both happened to carry the recessive genes for it. How could they know that would happen? How could you know you would have a deaf child. I think a big reason you ignored it is guilt, but it is not really your fault.
We grew up in the Detroit Metro area, and at the time- the 80's the West Bloomfield Schools were very good with deaf education. I don't know how they are now, but you might try to talk to someone there about how to proceed, especially if Ann Arbor is not helpful. I would also bet that Univ of Michigan has a good Audiology/speech pathology program, and they might be a good resource. I bet he could really benefit in the partial hearing ear having a hearing aid.
You need to work with professionals because there are many communication methods based on his level of hearing and your preference.
Do you know about Gallaudet University in Washington DC? I bet they have many resources for you.
I wish you luck, this is a challenge, but my sisters would tell you, it is not the end of the world. They are all happy people.