He can file modifications himself in family court. Many courts now have the forms online. You fill them out, send them in, they send you a packet, somethings you have to have her served (pay a company to do near her- $75 here, some places have the sheriff do it) and go to court on the date they sent you. The papers are free here to file, I have heard some places charging a small fee to file yourself.
You have to pick you battles but they are getting older so if you have some contact now, then maybe you can have contact with phones/emails/facebooks especially with the 13 year old. Send letters, cards for all (even little holidays). Contact their school and get online access or grades mailed to you, then send them a good job card/flowers.
Anything court ordered and she does not do, you can file contempt of court papers (same way as above). These papers are a little more serious and she has to go tell a judge why she is not following the court orders. (having email access, phone calls). Then you can ask for exact times/days this will happen.
He can try a registered letter (to prove he sent it).
Ex,
According to the court order dated XXX, it states the kids and I will have email access, phone calls....(quote that part). I have tried to contact them on XX dates and have left messages. Please have the kids call me weekly to see how they are doing. If you want to set up a day/time that I can talk to them each week, let me know when is good. Otherwise I will call every XX at XX. If they are not available, please have them call me within 24 hours. I really miss them and want to be a part of their lives as much as possible.
THank you
You
If she still refuses, (keep track, dates/times, keep phone bills) then you can send a stronger worded one that you will file contempt charges if you do not hear from the kids weekly.
It is hard to deal with a nutty ex, plus the distance does not help.