He's there to maintain the very unique culture of the company. I think it's sort of like if they have his son there he of all people should know Buffett's/Berkshire's culture and be able to best maintain it. He's got a few years to figure out a successor. But yeah this is mostly about the unique culture they have.
They can just add those people to the board. It’s not a set number and they don’t regularly add or remove people unless they think they can bring something to the table. Sometimes that is simply “holds a bajillion shares” but if they wanted a High Frequency Trading CEO on the board for insight into that, they could add both his kid and that CEO.
It's also entirely possible he's essentially competent at the job of being a board member which is a pretty do-little job if the CEO is doing a good job.
