Never take legal advice from the cops. Your son needs a lawyer. If he is indigent, he can get a public defender. If he's not, he'll need private counsel.

Being "to [SIC] tired" isn't sufficient to convict on DUI. The state will need to show some actual impairment. That can be the chemical tests as well as the results of the FSTs. Everybody thinks they handled the FSTs "no problem."

The lawyer will determine if there indeed was reasonable suspicion for the stop. If the fact he delayed going on green is the only thing they can come up with, that sounds like he may have a reasonable chance of questioning that.