Hey @Dannyd, great question! I find these types of questions quite compelling and interesting! I also have to resonate with @tsbehan about floundering with responding to a question. I find it especially to be the case with family when it seems so much more difficult to keep my frustration in check! So kudos to you on working hard to find good answers for your brother, and working through these tough questions with him!
I see at least three reasons why I find your brother’s line of thinking to be a bit lacking.
First, once again as Tim mentioned in his response, if morality is dictated by society, and there is no standard of morality external to society by which to determine right and wrong, then there is absolutely no ability to judge the moral standards of another society as right or wrong, i.e. we can’t condemn slavery in colonial England & America; we can’t condemn cannibalism in remote tribes; we can’t condemn infanticide in ancient cultures. All because we would be using our own societal standards as the basis for our condemnation! On the flip side, we can’t even commend acts of heroism and philanthropy in other societies which fly in the face of the standards established in those societies. I’m thinking of people imprisoned and punished for speaking out against a dictatorial and authoritarian governments. Our societal standards may say that their actions were good and just, but according to their societal standards, those actions were wrong, and ought to be reprimanded! In essence, I am saying that if the arbiter of morality is a given society’s standards, then we have no platform from which we can condemn wrong actions and praise right actions in a society other than the one in which we live.
Second, if we say that morality is determined by society, then we have no ability to say that there has been moral “progress.” I think that many people who hold to the idea of a societally-detemined standard of morality simultaneously think that we have had progress in our moral standards - i.e. abolition of slavery, civil rights movements, etc. However, if the society in which we live determines the moral standards, then we can in no way say that we have progressed morally. If we cannot say that it is better now that slavery is illegal here in the United States, I think that the statement of “morality is determined by the society in which you live” loses a great deal of its impetus and appeal.
And finally, I may need to do some more research into this, but it would seem to me as though societally-determined morality is offered as an alternative to a God-given standard of morality, but there doesn’t seem to be much logical force behind why it would be actually true or a more compelling argument. At the surface, it would seem plausible, but I don’t see any reason offered why I should in fact think it is in fact true. (Once again, please do point me to some research or thought that has been put into this that would contradict me on this point). The only reason that I see is the motivation to how we can conceive of morality without the existence of God, and this, I believe, is often the heart of this statement. Namely, I do not want God to exist so I would like to try to find an explanation for everything in a way that precludes the existence of God.
I hope these thoughts are helpful as you continue the dialogue with your brother! I know that it can be especially difficult to have these conversations with family, but I know that through the power of God and with the guidance of the Holy Spirit, you will be able to articulate well your thoughts on this with your brother. Praying for you!