According to the ECTIS form, anyone travelling to the US on the visa waiver must have a passport valid for six months after the date of return to the UK.
For travel to the UK, I agree that he will probably be stamped in for two weeks. The only issue I would worry about is what the airline policy is. Extend if you can.
Vicky
I may be wrong but I think the six months of validity also applies in the "opposite direction" travelling from the US to the UK, at least it did for me as a British citizen living as a permanent resident in the US.
Last year my passport was set to expire in September, but my travel date to fly to the UK was July. My memories of all the details are vague now but somewhere along the line, be it the airline, the travel agent, I'm not sure now, but SOMEONE in the industry told me I would not be permitted to board my plane to leave the country for overseas on a passport that had only two months validity left on it, regardless of when I thought I would be returning to the US (which was never; I was moving to the UK for good).
As stated, this was on a British passport but I got the impression that didn't matter -- my UK boyfriend also was told the six month rule for entering the US, so it seemed like from
either country the border wanted to see at least six months validity; it seemed to be the universal magic number regardless of which country's authority, and less than that meant you needed to renew your passport before travel.
Hope this helps for what it's worth. If I were the OP, I would go ahead and renew the son's passport even though it doesn't expire until two weeks after you have returned to the US.