I have an adult son with very different political views to my own and I don't see that as a problem. We have a lot of interesting discussions, he's very well informed, we learn from each other and help to nuance and deepen each others' views.
I would be worried about the empathy gap though. Making a statement like "black people can't be British" is not just ignorant; it also demonstrates a terrible lack of awareness of how it must feel to be someone, a citizen of their own country just as your son is, that has to live with people making those kind of duhumanising and erasing judgments every day of their life. For most people, that basic level of empathy would stop them from making such a statement, hopefully for long enough that they could actually then work out that it's not just cruel but also factually wrong.
Similarly with black Roman soldiers, the problem for me would be not the historical question of whether there were or weren't, but the question of why he thinks race is such an important marker of an individual that he can't overlook it. Did he scrutinize all the extras playing soldiers to check that they were an appropriate height for Roman soldiers? That they had the right hair colour? Art is not real, and I've never had a problem seeing people play people of different races. Or for that matter of different ages (eg in a youth theatre production) or whatever.
It's also worth considering the practical implications of the peculiar (to me) idea that while actors routinely play characters very different from themselves, in the one attribute of race they must always match up exactly. Are we saying that someone brought up in Britain, speaking English and part of British culture, who happens to be physically black, must ONLY EVER play one of the tiny minority of roles that corresponds to their race, even if they're the most fantastically talented and dedicated actor in the world with the potential to do great things across the entire reportoire? That seems insane to me, both for the artform and for human rights.
I'd be wanting to challenge him on these kinds of matters because they are issues of consideration and empathy, which he clearly needs to develop to function properly in the world both for himself and others. If as you say he might be on the autistic spectrum then there will obviously be some specific challenges involved in that. Whether he has right or left wing economic views, OTOH, I don't think you can or even should seek to determine. Just put your views as intelligently as possible and challenge him as you would anyone else.