Yes, it would be permissible to refer to or respond to a non-Muslim as a ‘brother’ referring to the common brotherhood of humanity.
According to some scholars, it would also be permissible to greet a non-Muslim with ‘Assalam ‘alaykum’ and respond to their greeting with ‘Wa’alaykum assalam’. The permission is more desirable if one hopes for their Islam. And Allah knows best.
[Sharh Sahih Muslim]
[Shaykh] Jamir Meah