Sounds like you're doing an amazing job and being incredibly supportive. It's hard it even impossible to say what's objectively rude though. Different people have different preferences. Personally, I quite like people filling in a word I'm struggling with. Other people hate it though. But it sounds like you and your bf have it all worked out. Perhaps something else to consider though - I hate when people start answering a question before I've finished it or otherwise advance the conversation before I've finished my bit. It makes me feel like people are impatient to finish the conversation and don't want to hear me out.