There really aren't any programs designed for PG preschoolers. The best you can hope for is a program that is flexible and welcoming. My DS went to Kindercare at that age. We had some trouble with a few people there, but one thing they offered which worked for him was a pullout for reading and math instruction (offered to any kids who were ready for it.) They also placed him in the K class shortly after he turned 4. Anyway, they were willing to be flexible, but some teachers there were not happy about the arrangements and made things unpleasant for DS. I would look for a situation where the people who will work with him every day are willing and enthusiastic about doing it.
Re testing: yes, there is WPPSI, but it is a very abbreviated test and results can vary a lot depending on the child's willingness to participate. At age 6, the WISC-IV can give you good information about strengths and weaknesses and a ballpark idea of level of giftedness.
