Such a place does exist; whether it is on the astral or in the mind of your friend, I can't say for sure as I've never dealt with Greek gods on the astral or otherwise. That all said, I will explain this about the astral; it is effected highly by your perception of it. For instance, if you expect that a certain being will look a certain way, they will, or if you expect them to do a certain thing, they can. Now, I'm not saying that said being is controlled by your perceptions or expectations, but rather you are controlled by them to believe that those perceptions and expectations are true. So, while your friend may believe all this is happening on the astral, it could be happening there, due to his mental state. Or, he could be working on a mental plane, and becoming confused between the two. A mental plane is simply a plane where your mind can wander and create its own world. Again, I don't know which is which as I have not met your friend and spoken to them of my knowledge.
About his status as a "astral son of Poseidon", I can't say. However, it is quite possible your friend was "claimed" by Poseidon as one of his worshippers or followers, with a title similar to son, but not exactly so. Again, he could have misinterpreted what Poseidon actually said and meant. But I can't say for sure.