What is the ruling on one who believes that beer is not an intoxicant (khamr)? And does he become a disbeliever by that?

If barley beer is alcohol-free, there is no harm in consuming it. However, if a large quantity of it intoxicates, then even a small quantity is forbidden. For someone to consider permissible the drinking of beer that is considered an intoxicant is a grave error, but it does not constitute apostasy. The forbidden intoxicant is that which intoxicates from grape juice, whether in small or large quantities, and it is the one whose permissibility, if claimed, leads to apostasy. As for other intoxicants, the majority of scholars also deem them forbidden, based on the Prophet's (peace be upon him) saying: "Every intoxicant is unlawful."