I am very sorry to hear about the difficulty your son will be facing
The vehicular offenses are unimportant in the face of the drug charge. They are misdemeanors and while they are certainly criminal offenses, they are not so serious as to significantly enhance the trouble he is in. Here is the statute
for the drug offense with which he has been charged. As you can see, if the amount is over 4 grams, this would be a felony of the first degree. A felony of the first degree in Texas has a minimum of 5 years and a maximum of a life sentence.
It is customary, though not guaranteed, that a DA will offer a plea to a reduced charge in trade for a plea offer. You can see in the statute that manufacturing and delivery as a first degree felony requires that he be found with anywhere from 4 grams to 200 grams. As there is a significant difference between possession of 9 grams and possession of 200, I believe that if your son wanted to dispose of the matter, the DA would likely offer him a plea to the felony in the second degree. The same felony in the second degree has a minimum of 2 years of prison and a maximum of 20.
If the DA is insistent on prison time, the offer would likely be something more than the 2 year minimum but less than the 5 year minimum on the first degree felony. I'm guessing but in my experience 3 or 3 1/2 sounds about right. However, as this is a non-violent offense and also a first arrest, he should be eligible for probation, on either the 2nd degree or the 1st degree charge. Unless he is a major player in a drug orgaization -- and the amount he has would not suggest that -- probation would be a real possibility. On a second degree felony this would be 2 to 10 years of probation.
Public defenders are experienced trial attorneys and the know the worth and weight of a case. If there are search issues, and he wants to go to trial and fight these charge, public defenders have plenty of experience trying and negotiating drug cases. If he is a heavy user as well, there may be some program-oriented dispositions that may be bargained for.
Everything will depend upon the particular facts and circumstances of the case, but probation would be a possibility.