What do you expect from volunteers?
If you want precise and immediate replies, consult local legal counsel.
In short, if your son is issued a subpoena to appear in court, he will have to go. If he fails to go he - and, perhaps, you - can be arrested. If you REALLY want to fight the subpoena, be prepared for him to miss additional school because you will have to hire an attorney who will seek to quash the subpoena ... not sure on what grounds, but he can probably come up with something that is threadbare but viable.
So, if you want to prevent him from responding to a subpoena he'll miss school anyway, more than likely, and it will cost you a bucket of money.
Your call.
- Carl