I think that was a very age appropriate answer. It's the truth, without the burden of adult sized details.
Ex in the other hand is not happy with me telling DS this as he thinks that telling him this will damage him
He hasn't shown himself to be a very good judge of damage assessment/control. The damage was done by EX the minute he chose the path of deception. Lying won't change or fix that, no matter how much he wishes it were so.
I've seen others recommend that statement as an age appropriate answer.
Children want to know that they are going to be OK, and have a tendency to assume blame for things they don't understand. Children understand the idea of cause and effect. When they don't have answers to their questions they can misinterpret themselves as being the "cause".
I think your honest, respectful answer will help DS process what is happening to his family in a healthier way.