Previous to this his speech was perfect and nothing traumatic has occurred. Keiran Stammering is best thought of as a problem of communication rather than simply speech which is a part of it.
Most stammerers can speak perfectly well, and manage to do so most of the time.
At two your son is, like all other two year olds, trying to get to grips with the incredibly complex and subtle ways in which we all communicate with each other. This will include learning a myriad of words but also many other aspects of communication, such as how to use those words, when to use them, when to use non-verbal communication, etc. It's an astonishing process and you must remember that it's going on at a time when he is also learning a million other things about the world around him and his relationship with it. It doesn't fail to amaze me how tiny children do pick all of this up and equally it's not surprising how many children have a few problems on the way. Research has shown that over half the children in the western world have signs of disfluency of speech at some time during the early years when they are learning to speak.