The Service Members Civil Relief Act (SCRA)
can allow his lawyer to make a "special appearance" on his behalf...so its certainly possible...particularly if they are still trying to do this "uncontested".
But it may not be in his best interest to do this...it really comes down to what they can agree on. IF they agree...then great, his lawyer can make the special appearance...life goes on.
BUT if they can NOT agree...then he may want to consider using the SCRA as a "shield" and invoking his right to a delay based on his deployment.
So again, this goes back to the question "can they agree on this"?
If she has her next wedding set this summer, and your son can demand a continuance under the SCRA, seems he has a bit of leverage. It may be he can use this to get the best result from the settlement.
As for the "surprises" in the court papers...yes, that is not uncommon...particularly one one side gets a lawyer. She can ask for what she wants...and your son can ask for what he wants...and they can pay their lawyers to fight this in front of the judge and let the judge decide...or more likely they will reach a settlement (again, after paying their lawyers).
As for the "order to pay"...if this is a court order to pay, then yes he needs to pay. You will know if its a court order if its signed by the judge or the clerk of the court.
If its not a court order to pay, he is not obliged to pay yet.
Let me know if you have more questions.