K., first thing I want to say about your son is PLEASE QUIT WORRYING!
Do you know that Einstein did not say a word until he turned FIVE years old?! Historical fact!
I also personally know the guy who went on ALL STRAIGHT "A"s in all the subjects in the University, he graduated with the degrees with highest honors, majoring in History and Philosophy, and he started talking only when he was 3 years old. I also talked to his parents, and they confirmed the fact.
Now, just remember that everybody has their own pace and own STYLE of growing into this world.
You have ways of communicating with your son, he understands you, you have this sign language developing, and he seems to like you talking to him, so do what is pleasant for him, and keep introducing him the world, and words, but do not worry and do not speed him up. He WILL talk when he will decide he needs to do that!
I believe what is important is for him to feel love, security, care, and safe happy family: provide him with this "must have" ingredients, and enjoy his presence in your life! All the rest will occur when the time is ripe for HIM :).
Myself, I was a weird kid: i didn't almost ever SMILE, until I was about 20 years old. I was not sad, I was contemplating a lot. I was thinking about many things. I also did not speak more than it would be absolutely necessary, like to pass me the salt, and to thank you for he present. I listened, but did not feel like talking. I also had a situation when my mom was concerned. Here is the story:
I lived with my Granny till i was 5, and she is Estonian, like my dad is. Now, my mom is Russian (she knew both languages), and at my age of 5 my parents too me into their home to live there permanently: mom started teaching me Russian. I listened, and nevdr responded, whereas my brother (3 years older) spoke both Estonian and Russian words often in the same sentence, mixing it all up into incomprehensible salad of words. As I did not respond, my mom got frustrated that i will never learn her Russian language, but the fact was that i didn't much speak Estonian either, i was just on thr receiving mode :). I LISTENED very carefully what mom said in russian, and then I silently pronounced her senrtences in my head. I remember it, i wanted to hear these words inside my head as clearly as i heard it from my mom, so i kept practicing them over in my head, but mom didn't know it as my lips didn't even move. Then, it might have been a day or too later, i went to mom, pulled on her skirt, and pronounced the whole sentence in Russian with a very clear Russian accent, and all, just perfect! and then, I was silent for several days again, practicing in my head some new expressions, or a short verse that my mom tried to introduce me with.
See?!
it's kind of strange, but I want to tell you, narrating the tale from my life, that everybody is unique! My brother blurted out everything, and slowly learned to say the words right, and I first practiced, and then presented the word as perfect as possible... we do not KNOW how the mind process of your son works, but there is obviously a hard work going on there. I'd suggest you just support his process of growing HIS OWN WAY by wrapping him into love and care, and please do not show any signs of worry, do not confuse him without reason, give him his time, just be his very bestest friend, and one day, you will know how imperative it is!
Wishing you to be patient, and extremely happy! All the best!
:)
M.