Let me tell you a similar story:
3 friends and me went kitesurfing from shore in Egypt to an island several kilometer away.
Sun was setting, so we decided to ride back.
2 went ahead and I was in the middle, and after about 1 km I noticed one buddy had his kite down...and no sign of the kite moving in any direction to relaunch it. Back then, we had bigger kites that were hard to relaunch (2004 I think).
So I went back to him and was shouting commands on how to relaunch (he was also experienced). But he didn't move at all ! No reaction whatsoever...just being dragged behind his kite, head up, but like mind absent, as you reported.
So I told him I will go to the kite and help relaunch. So I grab the kite and drag it to the wind window, trying to flip it onto its side...takes about 5-10 min until we managed it. By then he was acting again and got the kite up.
We then kited back and all is well. Once we landed the kite, he came to me and thanked me for saving his life...since he was not capable to do anything. Not sure whats the word for it, but maybe it is hypothermia. Guess it is a state of shock that not made him to anything.
Luckily the water is warm. But if it is colder water, and we would have lost sight of him (his kite was dark blue, almost impossible to spot it), things would have been way worse, like what happened to the 19 year old girl that died in Germany in 2007
Cold water is very dangerous, so be warned. I personally experienced how cold water can take your energy...I almost drowned too (that water had 16°C).
In your case grigorib already advised well. First take his board and keep an eye on him...once back, drag him in. If his condition is bad, forget the board and drag him in immediately.
If the kite makes problems, have him release it completely, and drag him in, get the kite later. Or someone else can help too with all these tasks.