1-He plays the guitar, as his brother does.
2-He plays the guitar, like his brother.
These sentences would normally mean:
a-He plays the guitar and so does his brother.
I think the sentences might also mean:
b-He plays the guitar the same way his brother does.
I always thought that in this case, the part after the comma has been added as an afterthought. I thought they could only reflect spoken language. But could these sentences not be used if we have two focal points? I want to say that he plays the guitar and also that he plays it like his brother. In that case, they would be acceptable in written formal English as well.