It's a strong cast! I found funny that Phoebe Nicholls is playing BC's mother again! She had a small role in Louis Wain. She is British so probably they aren't playing Americans but English living in NY.
I just checked Phoebe Nicholls’ filmography to see if she usually plays Americans (no) and guess what? She not just played BC’s mother in Louis Wain but also in Hawking! So this is the third time she plays his mother!
(Edited, I moved this post. Maybe I should move others)
I think it was first reported he was Sullivan (in a summary) but now he is Anderson! Sullivan is an Irish surname, probably more common in the USA than England (?). Anderson is Scottish. Maybe it fits with the idea that the character isn’t American after all. Of course, I’m speculating! I’m an ignorant! I don’t know anything! Lol
EDITED I actually checked and Anderson is more common in the USA than in England but still a common name! Lol
Yes Sullivan is a very common Irish surname. Most Sullivans tend to be from Cork which is in Munster province as you said RP. I’m sure there’s also lots of Sullivans in the US and the UK too because of Irish emigration to those places over the years.