Most Helpful Guy
Most Helpful Girl
... But could he not fully be over her...
You are in a date mate sticky situation with a newbie who is probably still googling and gawking over his Ex from the Past, which could turn into Not a Blast for you in your own relationship down the love line.
Some good advice right now, Now that you have Found out the facts, is go slow with Joe's flow, don't wear your own heart on your sleeve and perhaps it might not hurt you to sit him down and get on the same page.
Talk to him and find out how he might still feel for the skeleton in his closet. I believe he hasn't Found full closure and with you on his arm, you could be someone who is his rebound or even a somewhat comparison to the ghost of yesterday, which could rattle your bones a bit.
You don't want to start off on the wrong foot so nurse and nurture this with kid gloves and take it easy with baby steps for your own safety.
Good luck. xx