The guy has been completely hoodwinked by Kim Jong-il, and he is under political pressure to act tough. The initial response from him and his aide was a complete disbelief. His team and N. Koreans were working on the deal to have another inter-Korean summit and they are led to believe that the deal was almost sealed. There is local election in Jun. 2. I am sure he will stay tough until the election day. If his party wins, I don't know what his action afterward will be. If N. Korea presses on with other provocative moves(which is likely,) he has no choice but to remain tough.
In short, he can be made tough(on N. Korea) not by nature but by pressure from others.