In Demon Souls, in order to level up, I need to return to the Nexus. Can I return to the Nexus through the Archstone, in order to level up before fighting the boss?
You cannot return to the Nexus with all your souls unless you use the Archstone. There are multiple Archstones scattered throughout the world, so as long as you have access to one of them, you can return to level up.
There are also items called shard of Archstone which allow you to return to the Nexus from wherever you might be, without losing your souls.
So, to answer your question, if you can access an Archstone before the boss; yes, you can return to the Nexus before the boss fight.