health care proxy question

I dont know if anyone on here will know this but it’s worth a try to ask. My boyfriend and I been together for 7 years and we are engaged but because of COVID we decided to wait for a wedding. Anyways, my boyfriend doesn’t speak to his family, at all. They aren’t good people. And my boyfriend always fears of something happen to him they would obviously be able to make all of the choices and he knows they will not make the right ones. So he wants to do a health care proxy and make him his proxy. I know our primary doctor gives the forms out but im just wondering what exactly can a health care proxy do?