From what I understand, baby's can't really start signing back until around 6-7 months (because of hand control or something) and most don't start until quite a bit later. But they can totally understand you and respond in other ways a lot earlier. Like if you're signing milk a lot, you may find that he starts to show a lot of excitement whenever you sign it to him, because he knows food is on the way....lol!
I didn't start until later with Mackenzie (around 5-6 months I think?) And she didn't sign back until around maybe 9 months (I don't remember exactly....it is written in her baby calendar though!) but she definitely knew what I was doing. I found it hard to persevere with it though when she wasn't signing back....like I was thinking if it was really worth it. But once she started it was awesome! And I'm soooo glad that I kept going with it! We're still learning new signs all the time.
Christine you'll like this one...the most recent sign I taught her was "baseball" and now she says baseball and signs it whenever we're watching the game on tv!! You just put your two fists together off to the side as if you were holding a bat waiting for the pitch. Then you hit those two fists together a couple of times. 