How you are answering his questions are right on target. My parents explaining to me about my conditions helped me become who I am today. I started laser treatment when I was five and it was a eight hour trip one way for treatments. Being honest with your son about his conditions and explaining it to him at his level of age is the best you can do. Ignoring the problem and questions is the worst thing a parent could ever do and I am glad you are not taking the road. Children become way to mentally unstable and it gets worse as the child gets older. I know a few people with birthmarks that it has happen to.
Now for the comments/jokes, pointing, and staring. As he gets older it will get a lot worse. Teaching him how to not let these things get to him emotionally is your best option. Letting them roll off his shoulder for example so it will not eat him up inside. It will make him depressed somedays, and others he will come home crying. Trust me I have been down that road. Getting him active with your friends kids or neighber kids that are his age, and kids that go to school with him is the best way for him not to become an outcast. If you want to read more about social issues I did write a paper on it and it is on my website if you are interested www.hankspws.com
My family was always supportive of what ever I wanted to do. A lot of love also.
I hope this helps.