First person to say "he wasn't a public figure, they made him one" gets the fun job of making new law about it.
EDITS (replies to below):
Defamation laws in the US are actually very complex and in certain cases ambiguous. Different states have different standards for civil liability. In Gertz v. Robert Welch (https://en.wikipedia.org/wiki/Gertz_v._Robert_Welch,_Inc.), which involved a newspaper publishing lies about a non-public official, the Supreme Court held that state defamation laws had to have the plaintiff prove at the very least fault of some kind on the defendant's part, and that for punitive damages malice must be proved. The court also found that people cannot be made into a public official.
Certain statements are considered to be inherently damaging and fault does not need to be proved.
Regardless, clearly Ryan Lanza does not have a case since law enforcement officials did originally identify him, wrongly, as the suspect. The police were clearly not negligent because the reason they ID'd him as the gunman was because his ID was found on the gunman's body. I don't know if the news actually stated as a fact that he was guilty.
They did? The article made it sound like they used the name Ryan Lanza, but that they didn't identify which Ryan Lanza.
There's a very large legal industry surrounding shotgun legal claims in rapid succession throughout the country.
But winning a libel suit is no panacea — it doesn't magically make your life better. See e.g. http://en.wikipedia.org/wiki/Richard_Jewell#Libel_cases and http://en.wikipedia.org/wiki/Richard_Jewell#Aftermath .
Almost impossible to win a libel case in the USA, it will almost certainly be shot down on first amendment grounds or with him being/becoming a public figure after the police named him.
Otherwise, your assessment would be correct.