OP leaving aside that your DP's ex may or may not be a horrible person, your DP's relationship with his son is going to be extremely hard to maintain simply because he has chosen to move so far away. His son will always love him, but he will be hurt, upset, confused and maybe a little angry - and he has a right to be.
When my 2 were about his age my ex decided to go "home" for a year. He had his reasons, and they were valid, but home was on the other side of the world. It devastated my kids and DH and I were left to pick up the pieces. In the end he was only gone a year, but in that year they had lives that he simply wasn't part of, that he had no knowledge about, because he wasn't there. Phone calls can only make up for so much. Their relationship changed. They still loved him and were excited to have him back in the same country let-alone city, but the relationship was not the same.
Fast forward a few years and DH's ex decided to move 2 hours away from us. Might not sound like much, but again, the impact on his kids was huge. Because they were still at school they stayed with us during the week and we took them to his ex on a Friday, and picked them up on a Sunday. Except that they didn't want to do that every weekend, or even every second weekend, because they then couldn't hang out with their friends and be included in what their mates were doing, or, as Bluestich said, sometimes they were simply too tired and wanted to stay in their rooms and sleep/read/veg. Now that they are young adults, it's rare that they go to see their Mum. Not because they can't, but because their lives are in the city they've grown up in, which is not where she is.
I know you don't want to hear this, but the bottom line is that your DP has chosen to move a long way away from his son. Even without the complicating factor of a bitter or angry ex, his relationship with his son will suffer simply because he's not there. Being in a different city means that he will miss out on a huge amount. He can't go to school concerts, or sports days or parent/teacher interviews or pick his son up from a mate's place or help with homework - in other words the ordinary little things that happen every day that make up a life, he has chosen not to be part of.
You can blame his ex all you like, and certainly her anger won't be helping the situation, but your DP has created this problem by moving, he has to be the one to solve it.