While a drug test absolutely can be wrong, especially the presumptive tests you can use at home, it is probably more likely that he slipped up and took some suboxone.
But why is he bring drug tested? If you, or someone else in his life, is asking him to take these tests then I beg you to reconsider. It would be such a clear sign of a lack of trust that it's bound to breed resentment. It's also a huge stressor added at a time when he really needs a calm and supportive environment. Finally, giving a loved ones drug tests implies that they are getting clean because you asked them to - and that never works. He is not going to succeed in his recovery until he is doing it for himself.
Also, a relapse isn't a failure. It's part of the recovery process and if it's handled right it can be an opportunity to learn more about yourself. However, for that to happen your boyfriend needs to be able to talk honestly with you about any potential relapse
Finally, there really is an actual and not insignificant chance that the test was wrong. At my old clinic they frequently got false positives for suboxone in the urine dipstick tests, but the mass spec analysis would show these to be false positives.