Welcome! I remember that I was looking for preschools for my DS at that age to give myself a break! Those younger years can be exhausting. As others have said, it will probably be difficult to find a preschool program aimed at PG kids, and you probably won't find a place outside your own home that can feed your child's specific interests at his level at this point.
What are you looking to get out of preschool? For us, it was a break for mom, getting used to listening to an adult outside the family, and learning to get along with other kids. We ended up sending to our kiddo to a part-time preschool based on recommendations from friends. It turned out to be not the greatest fit, and in retrospect, we would have only gone one year instead of two (the 3yo year).
As for testing, there is also the SB-V, but most testers familiar with GT kids say to wait until you have a need for a test for a particular reason. In our case, we had our son tested at age 4 to see if the tester recommended early kindergarten. We decided to send our son to kindy with agemates and then skip first grade, but we were happy to have the testing and results before school to help with advocacy.
