Interesting question. Probably because he thinks that by talking to you about his child or mentioning his child, it will be a turnoff - which ironically is the exact opposite and he's effectively turning you off by not doing so.
As I've gotten older and because I have a child, I'll never understand people's latent desire to hide their kids existence from people they are interested in dating. That's need to know information like a mug. I don't have pictures of my daughter on my Facebook page but that's partially because I have created a higher profile for myself than I thought and frankly I don't need people that I don't know all up in my business and knowing who she is or what she looks like, etc. Plus, anybody who really knows me would not only have pictures but would know who her mother is and be able to see the plethora on her mother's Facebook page.
But the point is, anybody I deal with would know, upfront, in the first conversation, that I have a child so that they can decide if they'd like to go any further with me under those circumstances.
Any man who's attempting to keep such an important detail about his life away from you is somebody that you should be leary about trusting. It's his child...his flesh and blood. Who knows what else he might try to hide.
I agree with what BFF said BUT I know of someone who won't bring up their child until it gets serious because she doesn't view it as anything more than just casual dating. If she were serious about the guy she would tell him. Maybe that's what he is doing? He might just be very private and until you and he move to the next level he's going to keep you at a distance.