Bob, you forgot a very obvious suspect: Mordecai Grimes! Well, he coulda looked a whole lot different 14+ years earlier. Like Judy Tenuda says - it could happen!
[spoiler]Of course Quentin is Tad's father, Samantha took the chance of telling Quentin Tad was not his son in hopes he would let him go away with her more easily... little did she understand, Quentin knows a true Collins when he sees one. all male Collins' look like Tad and David when they are children..[/spoiler]