Anne,
That's a great question and one many grandparents and parents are now asking with autism showing up more and more on television, newspapers -- as well as the CDC reporting a significant increase in cases. My own daughter is a beautiful little girl with autism and I am now an autism specialist who helps parents when they first get the official diagnosis.
I would like agree with the other respondents: it is early to worry and it is common for children to begin talking at their own speed.
When children are very little, I am always more concerned about their RECEPTIVE language, in terms of possible autism diagnosis, than their EXPRESSIVE. In other words, as your grandson develops, it is more important that he comes to understand what you are saying and can follow your directions than whether he is an impressive talker. Any developmental chart will give you an idea of what he should be able to understand and do at approximately what age. I have seen many children who are very delayed in their speech, but will have no problem following directions you give them (running to get their shoes, finding the toy you are asking for, etc) and they eventually speak very well at a later age. Autistic kids often seem "deaf" early on and seem to have trouble hearing their name (consistently) or following a one-step command (though this is also not a hard-and-fast rule -- all autism is a spectrum).
It is also important to look at him socially. If he is very into interacting with people, is interested in other children and wants attention when he does something or sees something interesting (called joint attention), these are good signs.
Generally, professionals will be very leery of making any type of diagnosis of something as serious as autism until about the age of 2-1/2. There are a myriad of things that can be going on with speech that have nothing to do with such a diagnosis -- such as a real hearing problem or sensory processing issues -- so even if there is a concern, don't jump the gun and assume anything specific.
All states have free testing and provide early intervention for children 0-3. It never hurts to get a child tested and get a therapist in to work with him/her, so if you become more concerned, don't hesitate to bring up concerns with your pediatrician.