I think he should try to be as plain spoken as he can be. No attempts to bring too much gravitas, but also don't dude bro him for no reason either. I think it depends on the script too, as to what will feel right. If this is an Arthur that was not raised by an American father on the East Coast, then it can make sense that he doesn't sound like your buddy from down the street. Now, if that is what they are going with, the Arthur Curry Sr. raising him, then he can sound more contemporary I suppose.