Human form let’s him shapeshift.  It says so in the description of the power in the first line:  “you’re a shapeshifter”

I figured that was just a fluff description. Usually in other systems that part really isn't relevant to the feat, power, etc. and is sometimes even contradictory (looking at you 3.5). So it looks like I get to eat some crow and stand corrected unless someone else has a radically different interpretation. I thank everyone for their experience and help.

So I am about to start my first Dresden campaign as a GM. I have a player who wants to play a descendant (Scion) of a classical European red dragon. He wants to shapeshift from his human form into a dragon form. He is taking inhuman str, toughness, claws and channeling fire. He wants to take human form to get some refresh back but I said Human Form only works if you have Beast Change or some other shape shifting ability and that he could only transform between a human form and a "powered form" with a shapeshifting ability. Can anyone elaborate on how this is supposed to work?