6'8" person here. Even as height runs in the family, you don't know what will really happen. But there are other factors, too: how large is his current room? How large is his future room? (either different room in the house, or different house) What will he do for college? etc. A full size bed may be less optimal in some of these circumstances.
My advice would be not to over-think it. He's little now, and will be thrilled with a "big boy bed." Get the twin mattress, get it at a smart time. But a used frame. One tip: don't get a footboard. As was said before, bed lengths aren't different, and even "long" ones have limits. My feet have hung off the bed for...3 decades. I don't keep blankets on my feet, unless it's cold. And the first thing I do in a hotel room is tear out the tucked-in sheets. Otherwise, there is no scarring / deformity / noticeable consequence to it.