Words seem to vary so much at that age. He sounds like he's going well though. I also think you're bang on the money about him developing one skill at a time.

I'll give you an example of my range of experience. DD at that age could count to 10 independently and say the alphabet she had probably a hundred words. DS at that age could say dad, ruck (for truck), gog (dog), mum and 'arcy (our DD is Darcy)....that's it. He couldn't even mange to nod/shake his head for yes and no correctly...he knew that doing one illicited a response, but had no idea what meant what. He didn't repeat what we wanted him to and he either grunted, pointed or screamed. In the last 2 months has vocab has just exploded. He can ask for what he wants almost clearly and comes out with a new word almost every day that we've never heard.

They say that talking to them about anything and everything is the key.