Hi Karen,
My very best advice is this: let him learn at his own pace. Learning 'milestone' averages are just that - averages. Because your child does not match this profile does not mean that they are in any way disadvantaged, it just means that they approach things differently. It is very common for boys (particularly) to have delayed stages in learning, they make up for it later.
If you are particularly worried then you might be able to get some testing done at the MCHC to pick up any learning disablities, but at this age I'd be inclined to just relax and let your child learn at the pace they are comfortable with.

A friend of mine has a two-year-old boy who, until last month, did not speak AT ALL, even to say "Mum" or "Dad". He is a bright little spark and his comprehension is certainly there, but he feels absolutely no need to form words and communicates through pointing and saying "Ah".

Possibly one of the best ways to get him interested in speech is to read to him - a short, interesting, colourful book a couple of times a day. Make a pattern of reading him two books a day, one that changes each day and one that is the same for a week. Kids learn through repetition and love the same stories over and over, so this way you get both familiarity with the words and a new range of words each day.

Most of all, no matter what you do, don't expect instant results. Kids will develop at their own rate no matter how good a parent you are!

With love and encouragement,

Kerrie