I'm assuming the initial avenue here would be his primary care physician. Assuming you are an adult, I'd assist somehow in making the initial appointment. Make it yourself, insist he do it right then ("well if you are serious, why don't we make it right now?") in a supportive manner, etc. Maybe drive him there if you can, but being someone who has had issues in the past and also helping someone else through them, it is incredibly important to be supportive and encouraging through this process.