Given what you're saying, I would test him. It seems to me that it would only help to give you information about your ds and what he needs. As to what test I'm not sure, but can you find out what tests the school uses? I know our school has that info as part of their policy on gifted education. I realize the school may want to test again, but they would most likely be more willing to look at a test they knew about/ had heard of.
Good luck!