I think if you are only giving him one mare you would be better off waiting til he is three....to my way of thinking it has nothing to do with the physical maturity of the animal as a lot of colts prepared for the show ring are physically ready at one year old, let alone two.
I have always been happy to use a colt at two (and I have owned a lot and I have also dealt with hundreds, BTW) and none have ever come to any harm.
I would not just turn a colt out with mares at this age, but basically, if they are not mature they will not set the mares, if they are, they will.
Everything we do with our horses is "unnatural", so there is no point talking about what they do naturally.
Naturally, were a two year old colt to happen upon a herd of mares without a stallion he would breed as many as he possibly could before a mature stallion turned up and drove him off!!!
So long as you know what you are doing, and you take care he has no adverse experiences, it can be a good learning curve.
I have a coming two year old this year who I bred and who I shall be using this spring...he will breed at least three mares, of very differing types.
The point of this exercise is to remove the guesswork from "is he good enough to be kept entire" as to my way of thinking, winning everything in the ring means absolutely nothing if his foals are not up to scratch.
This way I can have foals on the ground that I can evaluate while he is still young enough to geld without it affecting him too badly, I find four year olds geld out badly, I would rather geld before two, but, in this case that is not possible.