Hello kdlee!
Welcome to the Chinese Genealogy forum! I also have Lee (李) ancestors from Guangdong Province, China, and will do my best to help you. However, the pieces of information that you currently have,
1. Your great-grandfather's name of Lee(李) Sing/Shing,
2. His being from Guangdong Province, China,
3. His estimated birth in 1899,
4. His migration with his siblings to the Philippines during the 1900's,
5. His leaving his parents in China since they were already old,
are not enough for searching for your Lee (李) relatives in China. Therefore, before you are provided any websites or links to look for your search, I must recommend that you first get more information about your Lee (李) ancestors, especially the following information in Chinese characters:
1. The full names of your ancestors,
2. The names of their exact places of birth or ancestry, exact places like villages or hamlets, or city or town neighborhoods, or even addresses.
The reasons that you need to obtain this information in Chinese characters are that:
1. China primarily uses Chinese characters,
2. Your Lee (李) ancestors in China most likely only used Chinese characters, and the ancestors who immigrated to the Philippines most likely still used Chinese characters,
3. Your Lee (李) relatives in China today most likely only use Chinese characters,
4. Chinese languages, such as Cantonese, have an extremely large number of homonyms - that is, words that sound the same - for instance, the character for your great-grandfather's given name "Sing" could be any one of 61 different characters that are pronounced "sing", with different tones, in Cantonese (per MDBG.net,
www.mdbg.net/chinese/dictionary?page=chardict&cdqcan=sing&cdqcanr=1).
The reason for obtaining the exact places of birth or ancestry is that Guangdong Province is a very large area containing many cities, districts, towns, and villages and hamlets.
You can try getting the additional information on your Lee (李) ancestors by:
1. Interviewing your Lee (李) elders about your Lee (李) ancestors - be sure to prepare appropriate questions for the interviews, and to record the interviews,
2. Getting original documents on, from, or to, your Lee (李) ancestors - documents such as birth certificates, burial records, immigration and naturalization documents, genealogical records, and personal letters,
3. Photographing the graves of your Lee (李) ancestors.
To reiterate, I recommend that you obtain additional information about Lee (李) ancestors, especially original text in Chinese characters.
By Lee (李) ancestors, I mean your direct Lee (李) ancestors and also their siblings and cousins.
I hope these recommendations help, and I wish you the best of luck!
LJ