Those people are ignorant they dont know what a mongolian blue spot is. My son has one too, its on his back and butt. Its babies from asian and african parents too. It disappears when they are a kid. I know a little african girl had a small one on her forehead, it went when she was 6. I dont think it should cause problems though, because its on his back/butt same as your sons, so its covered up usually. Only person going to see it is the health visitor when they weigh him. Doctors and nurses will be aware of what it is, and know it isnt a bruise.