Involuntary commitment will be hard in his situation. The easiest way to do that would be to call the cops if he does have an outburst of some type if he is threatening to hurt himself. Then it would be out of the family hands.
Just stay realistic so that you won't be frustrated if it doesn't happen. Maybe the family can get him to see rehab is the way.
That is lots of drama to be under pre-wedding so you are sounding stronger which is great.