There is a basic discussion over weeks on going about the nature of sin and two different theological positions, where I take the traditional Holiness movement view towards sin,
and they take the Free Grace theology view..
So, if I'm reading you correctly he is sinning because he has a "Free Grace" theological view in opposition to your view and therefore you can't call him a brother.
So would you also consider him an heretic?
I'm kind of reminded of the "log" and "speck" statement of Jesus here.