Hmmmm that is a toughy Roughquest!
How did he know where to find you?
Thing is you have no duty to anyone here but yourself, there is prob only so much you can tell his son anyway before your going round in circles so it may not be best for you to continue talking to his son if it has stirred up a lot of hurt and emotions.
As to why he covered up his first marriage? Only he knows why he did that! To be honest it can't have been for any good reasons can it? If I were you I would try to distance yourself from all of them before you get dragged back into the ex's life when he turns up wanting to know what you have said, why you told him this, that and the other etc..
No good can come from any of this, busy yourself so your not dwelling on him again and try not to understand why he did any of it as you will end up going crazy and will still be none the wiser.
You need to carry on as you were before the son turned up.
Good luck :o)