Joint sponsor files I-864. He does not need to be your relative, does not need to live in the same state nor in your legal residence to that matter. As long as he is citizen or legal permanent resident, lives in the USA and makes more than 125% of poverty level. I-864A is another form, if the person wishes to use somebody, for example a relative who lives in the same residence as you for at least six months prior to filing. This does not apply to your question.