Eunuch2be most of what I've read so far in this post (before mine) is incorrect.
Elastorators rings are designed for the castration of lambs and calves up to about the age of six weeks I think is the recommeded age, at which stage their nuts are way smaller than mine (about 1 8th the size) so it is size that increases the risk not the lack off. They are very good for castrating tom cats and dogs and I've never had a complication with those animals.
My family would have castrated hundreds of thousands of ram lambs and to my knowegde NEVER caused a death once or even had a complication. :smokes
As far as the pain goes, animals are way less sensitive to most sources, than are human. In my experience and I have posted this elswwhere, it is all over for them in ten to fifteen minuets and they are off to suckel mum and do what ever- mainly grow up to be fat & happy neauters. They would experienc less pain than a newborn baby boy being circumised.

But from time to time people abuse opportunities for what ever reason and I know of times when near adult or small balled adult males have been castrated this way and they experience unacceptable (in my opinion) levels of pain and are far more at risk. But they were successfully castrated.
I know of a bloke who was castrated with a rubber ring at his bucks party by his drunken mates, out in the western plains where I grew up and that was just tragic.

A man can be castrated this way without a doubt and it has happened but DON'T DO IT THIS WAY. It is designed for animal use and you never know what can go wrong.
You've already said that you are going to the good Doctor and that's the way to go:dong::dong::dong: