Unfortunately (for him) I'm pretty sure that he'd have to tunnel out before being able to attack due to never having a line of effect (you can't attack through walls), and that would provoke an attack of opportunity. The only exception I can think of would be with an incorporeal attack (or especially incorporeal reach attack) such as the phantom limb alchemist discovery. I'd personally say that ghost touch weapon would not work, unless it was to attack an incorporeal target, but incorporeal can't be sensed by tremorsense, making it useless.
I doubt there's any PC feat that allows a character to bypass that, but I'm sure many monsters have the ability to do so.
If you want this to somehow work for him, I'd look at various monster abilities that can pop out of the ground to attack, and then try to figure out a reasonable (likely as weak as possible) version of that which could be taken as a feat for him.
Well come to think about it, there are feats that mitigate or outright prevent taking AoOs while moving, so that could be used here to perfectly get what he wants I'd say. He'd have to tunnel in and out every time though, but I don't think that would be an issue (tunneling in and out just counts as regular movement as far as I know)
