How to help a friend with manic depression; trying to get him to start meds?

Common problem. When people are manic, they often don't see the need for intervention and believe that other people are unnecessarily complaining about them or prodding them. Most manic-depressive patients only seek help when they get depressed, but that is almost certain to follow a manic period. Education about the illness is essential. There are many popular books about the subject now.
He has to want it. Your friend has to want therapy and medication, and all you can really do is encourage him to seek both and engage deeply and meaningfully in the therapeutic process. You simply cannot do it for him, much as you'd like to. You can either be his friend or his therapist. You cannot be both, and even though you want to see him get better, trying to be both simply will not work.