My ex accuses the children of not loving him?

You can only do so much when it comes to reassuring your children, sadly the only person who can fix this is your ex, you need to speak to him about this behaviour because it is only going to effect your children's mental health.

There may be a chance he is saying this to make them say they don't love your SO or even grow to resent your SO, obviously that isn't going to work, he may end up driving the children to avoid your SO or reach out to your SO for the acceptance their father isn't giving them.

Either way unfortunately you can not undo all the damage he is doing, you could explain to your children about jealously and wanting to feel special, as basically this is what is driving your ex.

First of all, you have to understand that a lot of non-custodial biological parents feel this way. They feel like they are being replaced by the step-parent. Explain to your ex that you understand he feels like he is being replaced, but he needs to understand that NOBODY replaces Dad. Just doesn't happen. Your SO is more like an involved uncle to the kids. The problem is not the kids, it's the perception of the parent and you have to work on that or he may very well sour his children himself without realizing he is doing it.

It's him not them..he is the one with issues..he is the one creating this mess, not the children..he needs to be confronted and to stop his 'childish' behaviors..

the kids? you address the so who is acting like a kid. obviously he has a issue with them not being his kids tell him get a pair man up and enjoy them kids like his own the rest will take care of itself .