My DD had the Wippsi at 3.4 and it did not showcase reading, math, spelling, writing, or vocabulary anywhere near what she could do etc. She is very strong in those areas. She did well on the test, but I felt they were missing huge chunks of her ability and I don't mean just achievement things like reading or spelling, but creativity, understanding, etc. yet it is the test most places seem to want and I am sure your son would do well on it and you could always tell them about his other skills.
The test changes after 4, but I still don't think it tests reading at all. I don't know anything about the WAS: can you ask the schools directly what they would like to see?