What do you do when you suddenly find out that your partner/fiance, has kept a secret from you for 2.5 years?

I recently posted about my fiance's children and the amount of time he spends with them. (Thank you so much for the replies received already) Only the other day, did I find out that he actually has another child, which he kept secret from me! He had a relationship with a 26 year old woman, when he was 17, resulting in a pregnancy. He is now 45 and suddenly out of the blue, his son has recently made contact with him again. I hasten to add, that until then, he hadn't heard from or seen his son for at least 6 to 10 years(So I'm told). It came as a shock to him, as he never really bonded with his son, mainly due to himself being so young at the time, so he tried to obliterate it from his mind, shirking all responsibility. The fact that I only found out about it by chance, has upset me a great deal and I feel saddened and angry that he never confided in me in the first place, especially about an issue so relevant to his future wife! I'm questioning so much right now and my head is spinning. I'm not sure that I actually want to move away now, considering all his family will be on our doorstep, including his older son that I wasn't even aware of when I originally made my decision to sell my home and move away to be nearer his family! The other shock that I've had to deal with, is that his son informed him that he's about to be a granddad!! I feel that my fiance has lived a lie for the 2.5 years that I've known him, and the fact that he was aware of my previous relationship which was based on abuse and untruths, makes me feel even more of a victim and now unable to trust the one person who I thought I knew implicitly!

I just don't know what to think or do anymore. Any advice would be very welcome right now.