He will eventually see his grandfather. Better to prepare him for seeing his grandFATHER as a woman now than surprise him with it. Unless they plan to never see him again, they should tell him so he is not shocked. Give him time to let it sink in and ask any questions he may have.
Its not like waiting to tell this kid is going to prevent his life from becoming some drugged up train wreck anyways. The entire family is completely off their ass crazy and I am willing to bet decent money that this kid will be just as messed up as the rest of them one way or the other.
It is never good to hide the truth from anyone and children are no exception. As long as the child is old enough to comprehend what he or she is being told, then it is better to impart the information sooner rather than later. Without question, it is much better for the child to hear this sort of thing from his or her parents rather than other sources, not least when it involves people in the public eye.
Telling Mason now will prepare him more for what changes is about to happen. If Kourtney and Scott wait to tell him it will be harder on him because he will not understand why Grandpa is changing so much. It is best to explain now what exactly is going on and answer any further questions as the process progresses.