Travel Insurance for a Minor

DS is going to visit my brother in the US and I need to find a good travel insurance policy. So far I've filled in two online forms only to come to a stop when the traveller date of birth is entered - apparently there must be one traveller over the age of 18 with him for the policy to be issued.

Does anyone have any insurer recommendations? DS is 12 years old and will be travelling under the supervision of the airline staff.

Thanks in advance!

I'm sure I used the Post Office insurance a couple years ago.  Both girls were under 18 but one was 16.

Thanks for that, I don't know why I never think of the post office! I got a reasonable quote from them (I think, I've never bought travel ins. before) that I think I'll go with.