Very difficult. Adults need to come to their treatment voluntarily. Don't be co-dependent with an adult child with bipolar. Hold them accountable and don't give in to their pleas for financial assistance. They must be accountable for their actions. If they ask for help give it to them, but don't let them manipulate you. Bipolar patients are financial disasters. You also need to be aware of drug/alcohol use.
He must want help. It can be frustrating and disheartening to see a loved one suffering and yet be unwilling get help. However, the first step is for him to want to get better, to want help. With bipolar disorder, if he is predominantly manic, he may not see the need for help. If he is intent on harming himself, another person, or significant property, call 911 (a whole cascade of events will result, so plan ahead!).
Report. You have the ability to report your concerns to their psychiatrist. Although by HIPPA rules of privacy, the psychiatrist cannot divulge details to you unless it is a life threatening emergency. Also the psychiatrist needs to take any urgent concerns seriously and then considers rapid management perhaps hospitalizing. Anyone noticing risk of harm to self or others should call 911. Take care.