Hi, Pandora. I don't have much to say that others haven't already said, but I just want to post to let you know my story and lend additional support.
I first suspected I was gay when I was 11 and came out to my parents when I was 20. My dad was always supportive, but my mom seriously freaked out - accused me of joining a cult, etc. I remember just feeling numb. She has since come around, however, and now she is very supportive, even a little too supportive - outing me to people I probably wouldn't tell.
I hardly even remember her being not supportive because her more recent actions have erased her first reaction. I found out too late that some of the things she was struggling with I could have easily reassured her about if she had only told me. For instance, she thought I would never have kids, which is absolutely not true. I think it also helped her to realize that a lot of people you might think would be disapproving aren't. In the 8 yrs since I came out to my parents my entire extended family (who is very religious and unerringly republican) has been told (I left it up to my parents to decide when to tell them). There is not a single person who is not completely supportive of me and very accepting of my partner. We are even planning a wedding this year - something I never thought would be available to me. I can tell you that having the support of my family and especially my parents has made all the difference in the world to me.
I applaud you taking steps to be more supportive of your son, and encourage you to ask him questions when you have them, but if he isn't ready to answer, give him time. 17 is young - I distinctly remember not wanting to talk to my parents about anything at that point, even though I had a boyfriend then. You may want to consider telling him about how you feel about the actions you took when you were 15. Honesty is almost never wrong. As others have said, PFLAG can be a great source of information and support. It's similar to this website in a way. It's great to know there are other people out there in the same situation as you and you can talk to them about how they handled certain situations, etc.
Good luck. It will probably take effort, but I'm sure you can work things out with your son.