There are some serious problems here. The first is that spinal nerves do not regenerate - they are different from the nerves in the rest of your body which can somewhat self-repair.
Assuming you somehow get them to join, how on earth would you get them to join to the corresponding nerve on the other side of the gap? If they connect randomly you’d have a hell on earth of random twitches and false sensations.
Assuming you survive the procedure at all, you’re going to have to spend the rest of your life on immuno-suppressants so that the immune systems from the different parts don’t spend all their time fighting the other side. This part you could probably put up with.
There is the small matter that this has never been done on test animals, so there is has been no opportunity to get any good at it.
I do respect the head donor, he may be crazy brave, or crazy desperate, or just plain crazy, but I respect his courage that he would consider this when we haven’t done it with so much as a rat, or a monkey, or anything.