It's very likely that he is your real father. After all, why would some random older guy be searching for you on the internet, know all that information about you, and most importantly, know about your estranged dad whom you are forbidden t speak about? I mean, really, how many internet predator stories do you hear that start with "well, he told me he was my dad..."
That said, you still don't know anything about him. Even if your mom has been lying to you and keeping things from you for years, you don't actually know her real reason for keeping him from you. It could (maybe?) be for your own safety. If you do decide to meet up with him, I would strongly advise you to speak to your mom about it first, or *at least* tell a good friend or two what's going on so that *if* something happens (we can never be too safe), they'll know where to begin looking.
Hmmm lastly, I *do* think you should talk to your mom. I would start by telling her you need to talk to her in a serious tone and making sure she has a lot of time to devote to the discussion. I'm not sure, but I might be inclined to think she'd lie more if your stepdad was around. Then, I'd probably say "I know it makes you uncomfortable when I talk about this, and I've refrained for a really long time...but mom, I want you to understand that I need to know more about my father." If she starts getting mad and interrupting, say something like "Mom, it's too late for that. This could be a matter of my personal safety. I have already been in contact with him. I do want to meet him. But I want to know the truth first. Can you please answer some questions for me, honestly? I think I deserve to know after 20 years..."
lol sorry, I'm not asking you to memorize it, but yeah...just be mature I guess, and make it seem like a really serious matter that she can't brush off.
*edit* Although I see where Angelil is coming from in advising you not to do it at Christmas, this might be a conversation better left for face-to-face interaction. Phones are (imo) highly impersonal, and easy to walk away from. I think you would get a lot more out of talking about this face-to-face.