Why did you think he was a male in the first place? Was he popped male before you bought him? If it pops male its 100% male, if it doesn't pop male, doesn't mean its a female as males can "hold it in".

The biggest problem beginner poppers have IMO, is they place their thumb too close to the coacla so that they don't adequately "push" the hemipenes out as they start too far forward.