A: Yes, absolutely. As the parent, you are a member of any group that meets to decide services or placement for your son.
The law is clear regarding parent participation in meetings:
The parents of a child with a disability must be afforded an opportunity to participate in meetings with respect to -
(i) The identification, evaluation, and educational placement of the child; and
(ii) The provision of FAPE to the child. 34 CFR 300.501 (b)
Your son's IEP team is a good place to start and voice your concerns.
If you cannot make progress with the IEP team, using a special education advocate or lawyer may help. No matter how you proceed though, you should have a voice in the process.
A: Parents need not be present while school officials are asking questions of a student in most cases. However, if the questioning was by the police or your child was in police custody, constitutional protections may be triggered. You may want to consult a local criminal lawyer about the specific details of your case.
A: While you were at a school, this is more of a criminal law question and are likely being charged under your state's criminal code. Once your case goes to court, you can present whatever evidence you have during a trial. It may be helpful to contact a local criminal lawyer about your individual facts.