Why did he say "Lo Ish Besorah Atah ha'Yom"?
Malbim: Kings used to appoint someone to inform all happenings that day, whether good or bad. You were not appointed today, so you should not give bad news.
What is the meaning of "u'Visarta b'Yom Acher"?
Rashi: You will herald another salvation.
Malbim: This news includes praise of Hashem and His Hashgachah, for which one should bless. It is proper for a Kohen to herald it - you can do so at other times.
What is the meaning of "Ki Al Ken
Rashi: This is not a good matter for you to inform [David] about it, for the king's son died.
Why is "Ken" pronounced, but not written?
Radak: It is more correct without the word (do not inform about it, for the king's son died). It is pronounced to teach that it is proper that he died, for he rebelled against his father.