OP: Hopefully your son hired an attorney for advice regarding this situation. Other than the advice posted by Hanna, you should ignore the suggestions in this thread thus far.
There are several legal issues including whether or not your so has the right to submit a written response in lieu of a personal appearance.
The only broadly applicable advice that I can offer based on the limited information shared by you in this thread is:
Do NOT admit to anything;
Proffer a written response tailored to the two specific charges.
Only submit a writing if assisted by competent legal counsel (a lawyer licensed in that jurisdiction who has handled similar matters with this particular school).