That is good...that is, letters of support will be helpful. But more helpful if the letters come from officers...letters from other airmen will hold little weight in this matter.
Still, every bit helps.
What will likely happen?
It is up to his squadron commander. But what you describe, in most cases the AF will send the case to Art 15 and then, once the Art 15 is complete, will start the process for an administrative separation
for an Other Than Honorable (OTH) discharge
That happens in nearly all of the cases you describe.
Sometimes, if the airman has a poor reputation or a history of misconduct they will take them to court marital.
But if your son had a good record before this it is likely they will send him to Art 15 and then process him for an OTH...so he would not have a criminal conviction
Can he fight this?
Sure...if they want to separate him with an OTH they have to give him a hearing, and an attorney from ADC (JAG) to help him.
But the hearing is administrative in nature...and if there is evidence he used weed? Most likely he would be discharged.
The best thing he can do, by far, is hire an attorney with experience to assist. The JAG he is assigned will be energetic enough and will have no experience. Paying for an experienced attorney may help he remain in, or at least not get a poor discharge.
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.