Speaking as a man (I think I still am, though some days...), since you're the one who filed the D, he may be partially motivated to not appear weak or may be doing his routine in order to deal with the feelings of rejection (assuming he has them).
If that's the case, being casually sociable for the kid's sake but detached internally keeps you in front of him. If he gets over/past whatever he's feeling right now, you will still be there since the kid is so young. So there's plenty of time. But since this isn't going on your timeline, you can't get impatient!
I can tell you that despite feeling rejected in small amounts by my W for several years, her announcing she was leaving seriously intensified that feeling of rejection. Honestly, although I have moved forward and am in the process of moving on, it really, really hurts if I let it or dwell on it, and it doesn't feel like it will ever go away. I know it will, and I'm going there, but I'm not there yet.
Because of all this, I don't even want to see, hear, or be around my XW, and when I am, I have the urge to be either passive-aggressive, or aggressive-aggressive (which is more my style). I'm very agitated, uncomfortable, and annoyed.
Assuming he gets to the point where he's interested again, it will likely be a while. He needs time.
Keep in mind I'm not defending him. Just how I see it, and I may be projecting too much of how I feel right now on the sitch.