Since my son is left-handed, he naturally circled counterclockwise or widdershins while hubs and I circled clockwise, sunwise, or deiseal (I purposely did not use the Wiccan spelling). Did I hear a collective gasp? No, the magic did not seep from my sacred space and my ritual was not less special. Let me tell you why. Being a parent of a lefthander has given me new perspective on the world. Did you know that it is actually dangerous for a lefty to use some equipment (think: chainsaw). We live in a right-hand biased world. Magically speaking, it's the same way (the lefthand path vs the righthand path). I was reading a visualization where the reader was encouraged to make a ball of energy in her dominant hand. Dominant hand, not right hand. If he decides to follow paganism, I'm sure the Spirit won't mind if he does things "backwards".
As far as whether circle casting is necessary, I think it gets the mind in the proper focus for the ritual. My sabbat rituals are by no means solemn but they are respectful. The circle casting signals the beginning so that there is no last minute checking email or running to the bathroom. When I went to the Kingdom Hall, the meeting started with a song and prayer. It would seem kind of weird if the meeting was called to attention and then the first sermon started. Likewise the ritual ends with the circle being open.
Do you think it matters whether a circle is cast clock- or anticlockwise? Is circle casting even necessary? When is it mandatory or optional? Does this vary with a specific Pagan path?