Meg's answer hits it on the head regarding the fact that your son was a preemie. You really have to go by their due date for the first couple of years' milestones.
Additionally, it sounds like your son does understand what you're saying, as he responds to very simple commands ("look") and his name, not to mention that he's figured out how to consciously elicit a response from you.
How active is your son, and how are his motor skills? Children develop different skills at different speeds, so very often, they will excel at one thing (such as motor skills) and be "behind" on another. For example, my son is two and still doesn't talk verbally that much (generally just one or two words, doesn't say his own name, no pronouns, etc), but his motor skills are such that he not only signs, but can do things like climb ladders and stairs with ease.
One way to help increase communication between you and your son is through baby sign language. Children develop understanding long before their verbal skills, so signing can help them communicate (and can help ward off tantrums before they start).
Also, does your son go to day care or some other type of regular play groups with kids his age (or even a month or two older)? If not, it might be worth trying. Not only will it help him socialize, but may motivate him to work on his "weak" skills (one of the biggest things that got my son starting to verbally talk has been moving to the next room in daycare, where the kids are a little older and most of them talk better than he does, so he's started picking it up, too).