I think it depends why you want to test. It will give you more information about your DS6 that you'll find helpful. But like you said if things are going well at school, it may not be warranted at this time.
I'm not sure what state you are in, but here in PA a parent can request evaluation which will (by law) require an IQ test at the school's expense. Other less expensive options include getting tested at a local university where sometimes they have programs available for substantially less than private psychologists. (The test may be done by someone in training... it depends on the program.)
From memory (so I'd validate) IQs among siblings are generally within 10 pts of each other.
JB
