It's likely the marriage document will show that he was married before he married your mother. You will need those divorce papers and certificate of no appeal. The marriages after the one with your mother you can ignore.
If your mother is also applying (via a pre-1983 marriage), she will need to include her own divorce documents, and if your father is applying, all divorce documents must be submitted.
I spoke with them a few days ago. They need a certified divorce decree and certificate of no appeals. When I requested the divorce decree in Rhode Island, I mentioned to them what it was for and they brought the no appeals document to me without having to ask for it.