In Georgia we have a case directly on point. In short, a signal is not required to turn left from a left-turn-only lane because other motorists are on notice of what you plan to do. Such a stop should be found to violate the Fourth Amendment, and any evidence gathered therefrom should be suppressed. That should result in your case being dismissed.
No matter how easy or difficult the case may be to defend, keep in mind you are talking about your son facing a possible DUI conviction on his record for life. Think of job applications, graduate schools, etc. A lot of money is being invested in his college education to enhance his outlook for the future, think of hiring a lawyer as a similar (but hopefully smaller) investment in his future.
At a bac of 0.08, your son was probably not visibly impaired, so it may well be a very defensible case.
You absolutely need MD counsel to accurately answer your question, because this issue is state-specific. In GA where i practice, one of the elements the state must prove beyond a reasonable doubt is that you operated a MOVING vehicle while under the influence. But many states only require operation of the vehicle.