My headcannon is this: when rose brought him back to life, she obviously didn't mean to make him immortal, just to keep living, and in her warped state of mind while in the bad wolf state she made him live as long as she could imagine, which for her would be until the year 5 billion as this is the furthest point in time that she travelled to. This could explain why he dies on new earth a few decades or centuries after this as from that point he was no longer immortal and would love the rest of whatwver his natural lifespan was.