For this discussion I'm going to use the term Relativity of Morality to define the thought that each individual creates his/her/other morality.
If morality is relative, then anyone can believe anything.
Seems like a non sequitur
already, unless you tighten up the language a bit. You're jumping from the relativity of morality all the way to anyone believing anything. You would be clearer to say something like, 'then anyone can hold any moral belief.'
However, you're still misrepresenting moral relativity, which is a descriptive, not proscriptive of cultures
, not individuals. Moral relativity was never a statement that "anyone can believe anything." Instead, it means that various cultures can have conflicting norms and mores, ie morals, as a result of history.
It doesn't mean that whatever random idea one person pulls out of his bunghole today is just a good as any other idea. I think that if you clear up this misconception of what moral relativity means, then maybe you'll restructure the rest of your argument. Cheers.