my son is starting to come out of this phase. it was so nerve wracking and his pediatrician told me to ignore it and he can't do any damage to himself and it looks worse than it is. my first son did it too. it's for attention just tell yourself you've got a smart child bcuz they've paid enough attention to you to know they get attention when they get hurt so they think if they get "hurt" you'll give them the attention they want. kids will try to get any attention whether it's negative or positive you just have to ignore the stuff you don't like and praise the stuff that you do

but sometimes he cry

yup its attention. ignore him when he does it