How about actually writing a letter on paper and send it snail mail.
Boyfriend shares email with 50 yr old son. Boyfriend refuses to get his own email. We're having problems. I need to send personal letter otherwise he argues with everything I say. Too much to send by text. Should I send it if the son may read it?
No. It's a personal matter.
No .. I would never send something of such a personal nature knowing someone else WILL read it .. I would either mail it the conventional way (explaining why it's being sent that way) or I would say what I was trying say to his face .. or .. set up a private and free email account for him that we can use for private matters
Although there's a gender switch here somebody once offered this very good advice:
"Say it with diamonds, say it with mink; but never, ever say it with ink."
The guy's in his 70s, Anonymous. If you can't sort it out with him face-to-face you're going to have to write him off.
Either send it snail mail or just move on. There are obviously 3 people in your relationship, and if that is not what you want, it is time to move on.
Doing the math suggest your "boyfriend" is about 70 years old.
Personally, while I'm still pretty flexible at 71; but phone calls are pretty much the only thing I use my mobile phone for---I type way, way faster than I can text.
So don't ask me to text you, but I will email you.
For whatever reason, he's against having an email account for himself.
From what you say, he's unlikely to change.
Probably an indication that he doesn't have enough (or choses to not have enough) room in his life to accommodate you and your wishes.