Off the top of my head I'd say its going to be a long time in surgery...

. +1 for the pun.
So would the body reject the head or would the head reject the body
I`m guessing the body would reject the head, imagine the physiologic issues behind this transplant, fascinating stuff though.
The body would reject the head.
Certainly would be a painful way to die.
Afaik the only way to prevent it is to use immunosuppressants which would leave him open to disease as an added "benefit".
Even if the transplant is somehow successful I dont know how long he'd live even with anti rejection meds. An organ is one thing but surely the brain is well beyond the same difficulties.
The brain is swperated though by the blood brain barrier.
Im not sure if that would mean its protected from the immune system
. Although I imagine by that time he'd be dead already.The side effects of the drugs are part of the deal.
You talk about how long he'd live but have you seen his current situation? He's headed for a Hawking style functional head in a withered and crippled body.