Most times, if they planned everything, they already know they are out. A good chunk of the reason is he already has someone else and is too cowardly to tell you the truth. The guilty conscience sometimes makes them acquiesce to everything that is going on, planning marriage and such. But they do it because it would appear to be nicer than to tell you the truth.
I'm sorry this happened to you, but it is VERY fortunate this occurred before you guys tied the knot. It would be a pain in the butt to deal with a divorce and custody issues.. Is he going to continue supporting his child? Or did he check out of that too?