Visa for my father inlaw who was born in Xiamen

please advise what need to do. my father planned to visit his homeland as he is in his old age now. he was born in xiamen. went to the philippines during war with the whole family. And sad news, we just found out he is having problems for visa ( according to a travel agency) because he didn't finish the process of his naturalization way back then.. and he may not be able to go back to his homeland to have a glimpse. what is the best thing to do please advice. anyone please. my father inlaw wants to see