That's fascinating for us, and frustrating for Albert. On a practical level, does a text-to-speech app help him to communicate?
I'm not sure whether getting closer to English would improve his native speech, but there might be easier options than Swedish. For example, could you get him to mimic a strong Irish brogue, or an English-based creole like Jamaican Patois? Scots would be the most closely related language if you don't count that as a dialect. And historically, Frisian is closely related. Their shibboleth, which a Dutch speaker couldn't pronounce properly, is "Bûter, brea en griene tsiis."
I think dialects are not nuanced enough, they are essentially English. Plus he is a kind of set-in-his-ways sort of person, so even trying to get him to speak a few Swedish words, or carrying on in French is going to be difficult.
Cg