I thought the actor looked a bit like Jamie, so it worked for me. I know siblings don't always look alike, but in this case it helped sell the idea and convince the characters and the audience this really was Joe's son.
Also, I thought the mother was out of line. Her son wanted that job. He asked for it, because he didn't want a safe position. I like that the idea of Joe being able to make his own choices was echoed in the end. His mother didn't want him to go to family dinner, but ultimately, that wasn't her decision to make for him. Joe got to decide whether or not he wanted to be there.
My guess, and this is only a guess, is that it may also be one of those DNA tests that also tells you country too, so the students have to research a bit about what country/countries they're ancestors are from.
Overall, having three storylines instead of 4 was a huge improvement, but I did feel Jamie's was a bit rushed. I wished we could have seen more of the investigative work in figuring it all out instead of important information come as summary towards the end of the episode. I'm all for Joe being a regular at family dinner, but five plotlines per episode will kill this show.