There was no explanation for what happened to his character to explain why. It should be noted that Sebastian Cabot also missed nine episodes of the first season of Family Affair for “health reasons”.

I forgot about that as I haven’t watched season one in a couple years. He must have either cut or burned himself badly.

According to Wikipedia, he took a leave of absence to work on other tv shows, films and projects. Obviously as FA became a hit, he didn't do that again. :)