Is it rude or insulting to ask my new boyfriend to mind his English grammar and spelling?

I don't like when people write with bad English either. You'd be surprised how many people who speak English as their first language write badly, even on official documents. I think that the best way to handle it is to see how he feels about writing in English by mentioning how someone else you may know is having a tough time adjusting to the language especially writing in English. Listen to his response. He may reveal that he is having a hard time adjusting too, or that he doesn't care. Either way, as the conversation plays out, you'll know how to proceed. But remember, "be gentle" toward him as the conversation progresses. (2 Timothy 2:24)