I was hoping to hear back from you, but I'll provide an answer. If it doesn't address everything, reply on this question thread with what else you need to know or what you need clarified and I will add to my answer.
A subpoena is a court order, and if your son fails to respond to a court order, he can be fined or charged with contempt. As he's on parole, he wouldn't want that. HOWEVER, service has to be properly made or it can be ignored. He is supposed to be personally served with the subpoena by someone who is at least 18 and not a party to the action. So if he gets word of this in the mail, that's not proper service and he would not have to respond to it.
If he was properly served, he'd have to get permission from parole to return to Colorado indefinitely and they would have to approve that.
If he is supposed to testify for the state, he can call the prosecutor's office, explain his situation and let them know that not only wasn't he involved in this incident and didn't witness it but he is also without the financial wherewithal to afford the trip and to stay in Colorado. He can ask to be excused from the proceedings because it would create a financial hardship for him and he has nothing to contribute. If the state finds his testimony important, they will pay his expenses.
If he's a defense witness, he could contact the defense and proceed per above to ask to be excused from the proceedings.