I second the idea to attend a therapy appointment with him to discuss in a safe environment any concerns you may have and how you best can help him. It was a great help for my parents and oma when they went with me as they were able to find out how to help, what they were doing that was hindering my progress and therefore they needed to stop, and were able to understand the illness a lot better. Definitely be supportive. Don't express doubts, though you can approach him in a, "How can I help?" sort of way. He may need you to help him spot triggers or early symptoms of an episode if he knows what they are but isn't so great at spotting them yet. Changes in eating habits and sleeping habits are generally first signs for me, but each person is different. But you'll have to be careful not to seem judgmental or to always interpret something as a symptom rather than him and who he is. The therapy appointment would be a great help with figuring out this balance.
It would be worthwhile to be supportive of his recovery. Also you could speak to his therapist togethert about any concerns. NAMI friends and family support groups can be helpful as well as regards coping skills.