Wow guys calm down XD. The appropriate way would be to ask him when no ones around. Ask him in an appropriate manner if you make it worse that it already is the chances that well... The point is ask him if you really liked you try to stay with him.
This is nonsense. If he "likes" (is romantically interested in) another girl, he is NOT your BF. If he's asking you out, I question his interest in another girl. . I wonder ... is someone else telling you he "likes" a different gal? Perhaps you're being manipulated.