[我问他为什么,他说‘i don't know. i feel like doing it'].If you tell him about the consequence (小熊的眼睛 may become blind or 小熊 will be hurt and feel the pain) will he show different emotions? Or still emotionless? What you can do is perhaps some empathy training for him: http://parentingtheatriskchild.com/Empathy.html
since 现在医生都说孩子没有问题, you may not be too worried.