Santiago knew Emily and his kid ended up with Andres, who is someone he knew(and they had a mutual dislike, probably fought over something/someone before Emily) from college, it's a stretch to think he never knew his last name...
Then again he talked for an hour to a latino 18 year old boy, that looks a lot like him, and knows/"works" for Emily and he didn't say a word, i'd say it's consistent(ly dumb).
Everyone in this town hit their head as children...
On John-Erynn-Melissa:
That would be an easy way to figure out that Erynn is from Melissa... and i can see him asking her that sometime in the future...
But he's still unaware that "THE MOST GLORIOUS DAY OF HIS LIFE" resulted in a pregnancy.