It sounds like you are his side chick. The change when you are together and apart is the biggest sign. The dramatic decrease in contact means that after week 1, he opinion of you changed from potential gf to potential jump off. You guys moved WAY
too quickly. Another bad sign. I'm pretty sure he has not told his child's mother about you, because it is not serious. You are being played.
Do you mostly hang out in his or your house? Are you initiating things more often? Is he secretive/ trips up on lies? Do you know his full name?
All of this should have been worked out and perfect before you got to this point. In NO way is he going to stop messing with the child's mother for a woman he barely knows. They will share a bond for life.
You are not in love, you are infatuated. The sex and attention is good, and it's all fresh and new. Soon he won't contact you at all, and when you hang out it will be a booty call in his eyes. I been there done that, for 4 years, so I hope I can save you the trouble.
He is already playing games, please cut your losses, and move on.