This is how I view it from the information you have given.
He hasn't started divorce proceedings but to add you into the equation may complicate things for him in terms of settlements and/or seeing his son - in his mind
He hasn't introduced you to mutual friends as he is still married to his wife and it may be uncomfortable to do so
It took him 6 months before he introduced you to his parents and you haven't been back there since - even though you have met for dinner out. This could be because they felt uncomfortable and perhaps a displaced loyalty to his ex and their GS by having you visit their house. Maybe they aren't ready for him to have a GF
The FB thing, I understand. So many couples live out their lives on FB and it's too much. However, I wouldn't be surprised if he is friends with his ex on there - again, he doesn't want to rock the boat. Having said that, he could show you his profile to put your mind at rest, or at the very least, come up with a better reason than it's used for work.
I don't think he's being honest. I believe he wants to keep the status quo for an easy life because it works for him. BUT a relationship takes 2 people, it sounds like he is driving the way this relationship is to suit him.
Why did it take him so long to tell his ex about you?
Why haven't they started divorce proceedings? If things were settled the way they were, then I would think that as soon as his ex found out about you, she would start divorce proceedings?
What do you want? Are you truly happy to be a bystander in this relationship?