What I could see is that, since nap dropping, what really matters is not the length of the single night, but rather the average. So, F can do periods (few days or even one week or two) of shorter nights and longer days, then he compensates with stretches of longer nights and shorter days. His behavior is OK, so I think he is fine with this amount of sleep. And if you think about this, we are the same (at least I am), and we could have stretches of short nights if we can have a long night once a week. This was the way in which I coped with 6-7 h sleep before children (my sleep needs are more 8-9 h), doing a long catch up in the weekend...
So, I would not consider the minimum night length, but the average night length that he needs not to become OT. Now we are at less than 11 h (10.5-10.75), usually the minimum he does is 10 h (the maximum being 11.5 h), but we were out for few days camping and we had late nights and early days due to light coming in in the morning, and he dealt well with even the single 8.5-9.5 h night. He crashed for a short nap in the car though, a couple of times.
Since July, we also could shifted the usual time from 9 pm - 7.30-8 am to 10 pm - 8.30 -9 am, and we did it with a couple of nights at 9.30 pm and then jumping to 10 pm. He adjusted well, after a couple of earlier mornings. We are going back to the usual schedule now, as summer holidays are finished...