This question comes up a lot. I wish I knew how to search for it so I could just point you to another thread.
We use "a" before a proper noun to mean "some person by this name who was unknown to me."
If you and your mother know Mr. Brown, then the first is right.
If a man called for you and said "Please let him know that Mr. Brown called" and your mother did not know him, then the second sentence is right.
You need to leave a space after your punctuation marks. And before anyone else says it, I understand in the UK, the period after Mr., Dr. Mrs., etc. is not used as often, but they are absolutely required in the US.
Interested in Language