The stone is too heavy for Siela. Maybe when she levels up she will be able to move it? Maybe when a strong party member joins, he will be able to move it? How about mages? They can probably use magic spells to destroy the stone? I kept going back to the blocked paths or caves to see if I get lucky this time.
Nope, nothing worked.
In face, there is nothing you can do about it before you get to the part when you manage to help with the Queen. Once the Queen is healed, you then have a chance to finally meet this man in the cave in Gilbert's Path. He hides in the cave that's usually empty when you visit.
Bring a riceball with you when you visit the man. He will give you a pickaxe. Guess what it is for?
The man is there for another purpose. He will make you some equipment out of some items you show him later.