he is so young that he can’t dress himselfso的意思

he is so young that he can’t dress himself
so的意思

是so...that 句型,如此...以至于...
他如此的年轻以至于他不能自己穿衣服

so young 的意思是太小
The boy cannot dress (by) himself because he is too/so young.

so ,太怎样，so ~that太怎样以至于。。他太年轻了以至于自己不能穿衣

他是如此小以致他不能自己穿衣服