At what age or point in development is speech delay a concern?

DS2 is 19 months and says no words at all. But from an early age he started using hand signs or gestures to show me what he wanted. I didn't really encourage it, instead i'd say the words for what he wanted, though i did respond to them. Examples are; opening and closing his hand for what he wants, laying a flat palm when he wants his hand wiped, nodding for yes, shaking head for no, waving, pointing. Now he does other things like getting a nappy when he wants to be changed or giving me the remote control if he wants to watch a dvd. He clearly understands everything because he follows all our instructions, eg; put your cup on the blue table, we're going out now (he'll go and get his shoes). He also does a lot on his own accord such as bringing an empty plate to the kitchen, putting toys away or getting things. He just does not respond with words, i'll ask if he wants more or likes something and he'll nod or shake his head. For the past few months we've made more of an effort to speak clearly and state words when he makes the signs but he just doesn't seem interested in speaking. The only sounds he makes are "ahh" and "ba", other than that he's pretty much silent, though he is constantly doing something, playing, exploring, he's never really still, very different from DS1, who spoke early and is still a talker rather than a doer.

Doing some research he seems behind for his age as opposed to actually saying words but intelligence wise on track because of the understanding and follow through. I have tried over exaggerating the mouth movements for basic words "mum", "dad", "milk", "water", but he just laughs. He doesn't try to imitate me, he also isn't concerned about the words because he gets whatever he needs by non verbal communication. We are wondering if he's lazy as in not having to talk or whether this is something we should start looking at.

He's 19 months, should i wait a while longer or take him to the GP? a specialist?