1. It's not Patrick. (Yes, it is really not Patrick)
2. It's not his father (RJ did this because he was "slandered in the media").
3. It's not a woman. (it should be obvious, but for example: RJ slept with Rosalind and I don't think she's lesbian)
4. We have already seen him.
5. A powerful and influential person.

So I'm going for Virgil Minelli and my secret favourite is Walter Mashburn.
Bret Stiles just seems to have a good relationship with RJ.

In the end the truth will come and set us free