As a Born Again Baptisted Christian I too believe your Dad is indeed in Heaven. Earth is temporary, Heaven is forever.
I am a retired alcohol/drug councelor. Addiction is a tough thing for most to overcome. Some addicts I am personally aware of (my wife included) do recover and remain clean. This is the hope yout Dad saw. He held to this hope. We all should. Hope is a good thing.
My thoughts and prayers are with you and your's.