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I always recommend hiring a lawyer when there is a potential for criminal liability.
While he may not have any issues with a DUI based on alcohol consumption (you say he blew a 0.0), the blood and urine samples test for other substances as well. Even if he is found not to have been driving under the influence, if he is still on probation, he may have a violation of probation issue.
Hopefully hiring counsel is only precautionary, but this is probably a good idea (there is probably a reason why he was asked to give urine and blood samples even though he didn't have a positive breathalizer test).
Your son should speak with his defense attorney, but in general, do not speak with law enforcement unless your attorney is present (even "informal" questioning can lead to incriminating statements).