It feels like there's something missing in their story. Why would you testify against your 93 year old father that late in his life? You're pretty much sentencing him to death (even though we learn that he served his sentence and was released).
There must have been something the son got out of collaborating with the FBI besides just doing the right thing, and we're not getting the full story.
Looks like he was down on his luck, tired of being a mobster he never wanted to be and wanted out. The FBI agent gave him that out he wanted. That’s what it looks like. There could be more as you suggest, but given his life on the periphery, it doesn’t look like there was money involved.
There must have been something the son got out of collaborating with the FBI besides just doing the right thing, and we're not getting the full story.