So there's receptive vocabulary and expressive vocabulary. Receptive is how many words you can understand, expressive is how many you can speak.
Receptive vocab develops much earlier than expressive - infants & toddlers can understand a lot more words than they can speak, because, well, speaking is complicated! It requires the coordination and maturation of brain, facial muscles, throat muscles, diaphragm, tongue, teeth... its a lot to master.
If his receptive vocabulary is fairly big - if you ask him to point at something, or to get it, or you ask him questions he can answer (maybe with a nod, or a number of fingers) then it is less likely that it is a problem. Some children are just slower to develop an expressive vocabulary - maybe a little bit of apraxia, maybe just a lot of the self-conscious emotions, or maybe neither - and then they have their "vocabulary spurt" (when most children begin to pick up in expressive vocab, around 18mo) a bit later. But if his receptive vocabulary is small - he doesn't seem to understand when you say what should be familiar words, like bottle or mommy or nap, then you should definitely take him in for an assessment.
Like KinttedJimmyBoos said, this is the age when some developmental issues begin to become apparent, just because of how the infant brain typically develops. Typically, though - there are lots of differences between individuals of any age, so there's no reason to panic yet.
I hope this helped, and I apologize for any typos.