Found these links...

New this month: Speaking volumes

By 15 months, the majority (about 75 percent) of children have a vocabulary that consists of "Mama" and "Dada" plus at least three other words, usually nouns, such as "cookie," "ball," and "dog." "More," and of course the all-time toddler favorite, "No!" are also common early words. A typical 15-month-old can also follow simple commands, such as "Bring me your shoes," or "Put the book down." She also understands the meanings of phrases such as "No," "Come here," "Show me," and "Look."

http://www.babycenter.com/0_your-15-month-olds-language-and-cognitive-develop...

I found this link too:  http://www.childdevelopmentinfo.com/development/language_development.shtml

Sounds like your little guy is meeting his verbal milestones.   I would stop worrying unless there are other indications that make you believe he's delayed developmentally.

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