Well, no one really knows any reason why stammers take place. All we can do is note that the stammer exists, and, if it so happens to be the case in this situation, note that stammering was not an issue until a particular event took place.
It is truly not important at all to "find the cause", so much as it is to note the symptom and the facts surrounding it (when did it start? what makes it worse or better? what sensations occur with it? etc. etc. etc.).

You could wait to see if it dissipates as he grows, or take up a conventional treatment for it. It's up to you. But if you want to treat it homeopathically, take your son to a homeopath for a consultation and a constitutional remedy and forget about prescribing potentized medicine for the condition on your own.

__________________
...and deliverance has many faces<br />but grace<br />is an aquaintance of mine