Verified answer

You have a long way to go before you can head towards Sunyshore City. Before that guy will leave, you have to earn the other seven Sinnoh Gym Badges and stop Team Galactic's nefarious plot. Make a mental note of where this guy is, and come back to it later.

according to my guide, you have to restore everything so the black out in Sunyshore City is gone. the few pages in my guide before Sunyshore show beating Team Galactic's Boss Cyrus(located in Veilstone City) and defeating or catching Giratina. have you done all of this? if so, it seems like the guy should be gone. maybe you missed something? if you have done any of this, then do it and he should be gone:) i hope this helps. i myself haven't finish Platinum yet, that is why my answer sounds odd.