I'm going to throw my opinion out there, but before I do so I am asking that others be respectful, as my suggestions are going to differ from theirs. I am a Christian and I hate/fear being judged by those who do not know me based on the idiot representation that Christians have. Also, I will be using the term gay in place of homosexuals only because it is faster and easier to type. I mean no disrespect to anyone.

Also, I wonder what Your(original posters) beliefs are concerning homosexuality given your Catholic background.

I personally believe most gays are born gay, but I also believe it is a sin to act on these feelings. This is similar to my belief that I am born an alcoholic, but it is my responsibility to not give in to my cravings/desires. If you don't agree with me, I don't care, I'm not here to argue it, and neither of us will change each others minds on the subject so please don't turn this into that. I'm just giving background to explain what I would choose as a coarse of action (if I were in your place).

I would sit my son down privately, and explain to him my beliefs and why I hold them (biblical backing, expeiential backing, ect.)
Next, I would tell him that I love him no matter what he chooses. I'm not going to shy away or treat him any differently than I would if he were heterosexual. I will show equal respect to him and any partners he chooses to bring home. I'm not embarrassed or ashamed of him. I believe he is going in a direction that God does not want him to, but that is the last I will bring up of it, unless he inquires further about my beliefs.
I would tell him that I really do love him and want him to know that he can come to me and talk about anything. I will listen and try to offer emotional support as best I can. I would tell him that I am sorry I didn't notice anything was going on sooner. And I know that this is going to be very difficult for him, whichever course of action he chooses to take. I would apologize for the way others are going to react and encourage him to let me know if he needed help.