Oral hygiene

Hello there, I'm here to seek some advice about my boyfriend. English is not my first language, so please bear with me.

My boyfriend has uneven teeth, and his breath smells terrible. Almost all of his teeth are covered in tartar. Once, he playfully bit my cheek (I have chubby cheeks), and afterward, my cheek smelled so bad that I had to spray body mist on my face to get rid of the odor.

I’ve talked to him about his bad breath before, but he claims it’s due to a stomach problem that causes his mouth to smell. Honestly, I don’t buy that excuse. I’m not sure what I can do to make him take his oral hygiene seriously.