I'm sure others have already addressed the most obvious here in that boys and girls are *vastly* different, especially when it comes to verbal expression. This continues throughout their entire lives - I think I read somewhere that women use, on average, 30,000 words per day and men use, on average, around 5,000. I don't recall the exact numbers, but it was something that dramatic, literally.
I have nothing but boys, but I remember having the same fears with my oldest son because I have a niece who is 3 weeks older than him. She was full of chatter, make-believe, dialogue, etc. at the earliest possible stages while my son was nearly mute. I even mentioned it to my pediatrician b/c I was so concerned. Turns out, he's still a boy of few words - he's 13 now and an honor student. He, like your son, understood everything I said which you should take as a comfort - if he didn't understand you, that would be cause for genuine concern.
Hang in there and maybe consider reading the book "Bringing up Boys" by Dr. James Dobson. It's a real eye opener for us mommies with sons!