Assuming your son is over the age of 18, he should be the one making this decision. And, he should be the one writing the letter. You should support him... but you shouldn't be doing this for him.
There are a couple of reasons for this, the most important being that your son needs to learn to fend for himself as an adult. I don't think I would advise him to put his scrape with the law in any appeal. I am not sure if academic suspension can even be appealled... you get bad grades and the suspension is automatic I think.
The school has an advising department which will help him. He should be able to find a counselor who will advocate for him, and tell him what he can do. One option may for him to take a lightened load (less than full time) to prove that he can do the work... and then go back to full time in a semester or two.