Read this. Hire a good lawyer. Question the contamination control of the lab who performed the test, the training of person who performed the sampling, question their analytical methods, how accurate are their results, is it not possible to get a false positive result, to get contamination from other sources?
One crystal of TNT, for example, contains 30 ng of TNT which can be detected easily by a method called GCMS. Just one crystal! How do we know contamination didn’t occur from other sources in the lab??
Your brother is safe if all they could find was explosive residue on his hands. If your brother has no criminal history and if he wasn’t involved in any suspicious activities previously, there’s no way they can charge him.
He’s innocent until proven guilty, which can’t be done on the basis of FR residue alone.