A rather unlikely story with some even less likely coincidences, but some very fine acting indeed, particularly from David Harper, who handles his first starring role brilliantly.
Yes, the runaway in question is Jim Bob. Having been teased relentlessly for bringing a guinea pig home from school, he decides to leave home when the creature expires unexpectedly. Despite the serious theme, the whole thing is rather lighthearted, with the family viewing his departure more as an inconvenience than anything else.
Thanks to David's excellent performance, I did feel very sorry for Jim Bob. Ben and Jason were pretty merciless in their teasing, and our hero looked very hurt indeed. I loved that he kept running away from John-boy and I laughed out loud at the scene in the recruiting office. Great writing and lovely acting.
John-boy gets a subplot in which he misses going to a guest lecture with a girl named Amy. This fails on account of two ridiculous coincidences. Firstly, John-boy 'just happens' to bump into the guest lecturer when he is in the cafe with Jim Bob, then he 'just happens' to bump into Amy on campus as his family is driving off. Watch it and you'll see what I mean.
I'm not sure if it's intentional, but Jim Bob does seem to get short shrift compared to his siblings. When Elizabeth went missing in the opening episode, there was a full-on manhunt. Here, John is more concerned about fulfilling an order. Jim Bob sums it up perfectly when he signs his leaving note 'one of your children, Jim Bob Walton'.
Lastly, Jim Bob gives his date of birth as 13th June 1924 while at the recruiting office. This would make him only ten years old when the episode takes place (in the autumn of 1934), despite David Harper being almost thirteen at the time.