My wife and I have a 18-month old son. I am (as well as my son!) a cohen. We have noticed that he likes to use his left hand more than his right. I vaguely remember that this can be a problem vis-a-vis performing the avoda. Is this considered a "mum" or any other disqualification for the Kahuna? Is the disquaification only for the Kohen Gadol with his intricate avodah?

Avodah - sacrificial acts in the Temple - must be performed with the right hand by righties. There is an opinion that lefties are considered as having a mum, while Rashi seems to say that performance of these acts with the left hand is not the proper "way" that is it does not honor the Temple.