If someone has good knowledge but not a good writer, can he become a blogger?

It does not follow. A person may be well-informed in a variety of topics/issues, but it does not necessarily mean that he would want to write down everything he knows about. There are people who rather love to talk than to scribble their thoughts. 

I agree with experts answers...... Thank you for invitation

Blogging is basically about creating content for the web that your readers will find appealing to read. Writing is expression basically. You need ideas, thoughts and a sense of expression to be able to write so well that you capture your audience. 

Once you figure out that your thoughts and the knowledge that you have does attract an audience and you know people who can relate to it, you can definitely blog about it. Yes it is very important to be able to write well but gradually as you keep writing you will get better. 

So yes, you can blog even if you're not a very good write, the main idea is to put out your thoughts out there basically , but on the other hand you also want as much discussion on your post and read what others have to say about what you think as well. For that to happen you need to be able to communicate i.e write well and write clearly, and blogging infact is a great way to start off that.

Hope my answer helped!

He can, writing a few posts he will get into a habit of correcting his mistakes. That and his audience can get accustomed to his style the more they read and like his content.