Here is my take on that sign, and as it was pointed out that the TMUTCD used the word "guidance" and I will take justalayman word for that since I have not read it. However, usually in the federal MUTCD which all states are mandated to follow sign placement is not a guidance especially one that ticket can be written for, they usually have to conform to the standards and many times they state where they must be placed and what other information can be included with a sign, like time restrictions, however, I am not sure I ever seen from a ramp so that alone my not be valid.
However, that sign seems a bit confusing at best, obviously you can not make a left turn anywhere around where the sign is placed since the road is divided. However, 500 ft down the left most lane can make a left turn, so unless your son made a left turn from that right lane (ramp) then he did nothing wrong.
However, base on the fact they have a solid white line from the ramp to the intersection tells me you can not ever change lanes until you pass the intersection. Since I did not read the other post about this subject, I am assuming your son probably came down the ramp, crossed the solid white line then proceeded to the left lane and made a left turn, or at least moved to one of those left most lanes and proceed. In which case he did not make a left turn form the ramp lane either.
If that is what happen I would say the state was attempting to keep traffic on the ramp on the right most lane until after the intersection and attempted to do so by putting up a no left turn from ramp sign since most people cross solid white lines all the time and thought that people would understand that if you came down the ramp you can not move left to make the left turn at the intersection.