cybercat,
I too wondered when seeing those most recent labs they did on him, if he might be on slightly too high a dose but the range on that FT4 shows high-normal and not flagged high. With levothyroxine brands, they can do 12mcg dose changes which is small but helps tweak it. His Dr. will likely tweak it down a bit if the next blood retest shows the T4 to be flagged slightly above normal.
If you feel he should be rechecked a little sooner than 6-months, you might discuss with him wanting the thyroid ones reordered at two month entervals for a while longer. I would bet the Dr. will agree to it. Six months is reasonable in patients whom a Dr. feels has reached their proper dose level. Your son's case is a more complicated situation and for your own comfort, it's understandable you would like to see a shorter time between retests, at least for now.