Here's something I found online:
From 18 to 24 Months
Kids should have a vocabulary of about 20 words by 18 months and 50 or more partial words by the time they turn 2. By age 2, kids should be learning to combine two words, such as "baby crying" or "Daddy big." A 2-year-old should also be able to follow two-step commands (such as "Please pick up the toy and bring me your cup").
From 2 to 3 Years
Parents often witness an "explosion" in their child's speech. Your toddler's vocabulary should increase (to too many words to count) and he or she should routinely combine three or more words into sentences.
Comprehension also should increase — by 3 years of age, a child should begin to understand what it means to "put it on the table" or "put it under the bed." Your child also should begin to identify colors and comprehend descriptive concepts (big versus little, for example).
You can also go to ASHA.ORG and click on the child speech and language link. This is the American Speech-Language-Hearing association website. They have a public link and the top and you can futher click on normal speech development link.
I work in rehab and am familiar with speech therapy. There is nothing wrong with having to refer your child to such a service. You would rather do it now than when he's in school and that much farther behind. Usually the therapist can work with the 2 of you a couple of days a week for 1+ months, depending upon the delay...I know this is not what you want to hear, but he should be saying much more!