If he immigrated legally you might be able to gain access to his files by submitting a FOIA (Freedom of Information Act) request to the USCIS, formerly known as the INS.
Start by going to a good library that can provide you with information to the contact folks under USCIS. Find some one with access to the 1980-1990 data. Approximately, 20,000-40,000 dominicans entered the US at that time. Filter out how many Hector Santo's were there. Go from there...
To help in your search you need to narrow down his date of birth. This will help you immensely in your search.
Also try other individuals that resided in the same neighborhood as he did in DR that might be able to provide you with more specific information.
Good Luck!
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