what are you asking here?!
you can insist to drag his *buttock* to the nearest clinic and request a full battery of drug-tests, but he's 26 and not a child anymore.. if he is being treated for a Bipolar disorder, he should be able to show you his meds (such as mood-stabilizers, antidepressants and anti-psychotic meds) that can only be prescribed by medical doctors..
if you are 100% sure that he is using.. then an 'intervention' is what you need to do..
you can Google or Yahoo-search on how to conduct such interventions which require everyone's participation and having a solution at hand.. for example (and you can find more examples online): contact everyone in his life behind his back, inform them what is going on, and set a date for the intervention meeting.. in that surprise meeting (kinda like a surprise Birthday party), everyone reads what he/she wrote about their concerns and worries.. and at the end of the intervention, make sure that you have already set up a meeting with a therapist or a clinic.. make sure you already have a solution for him at the intervention..
all the best..