R.
Diana,
My thinking is to just wait until your child is in kindergarten. It's really hard to test IQ in the preschool years. He can be evalulated by a school psychologist at age 5--if you request. If your child is truly gifted, it won't just go away. I doubt that his potential will be "wasted". Just keep doing what you're doing. Give him lots of opportunities for stimulation (reading, trips to museums).
Also, think about what it is that you want to do with the information if he is "gifted". Would you want him in a special school? True giftedness is very rare and generally runs in families. Also, many truly gifted children have a great deal of difficulty understanding social cues and have problems making friends. Help in these areas is really important as it's a necessary life skill.
Good luck!