SWIM = "Someone Who Is Me"?

So the probation officer has been made aware of this drug test.

If there's any truth to SWIM's effort to blame his drug test results on OTC medications, which I doubt, he should consider getting a drug test from a clinic that will confirm his identity to the satisfaction of the court, and perform a drug test that is sufficiently specific that it proves that he did not use illegal drugs. If there's a remaining sample from the drug test that was performed, it may be possible to get a more specific test of that sample. Note that if the probation department isn't satisfied that it was SWIM who got the drug test (as opposed to somebody using his name), they are likely to tell him that a clean result is meaningless.

Beyond that, SWIM should talk to his lawyer.