K8eee I wish I had a magic formula to send to you on how to magic up a great relationship with your partner's ex, unfortunately it is total pot luck as to what kind of person the ex turns out to be. It's regardless of how much effort, respect, time or money you spend trying to build things up.
Either the ex is someone who understands what it means to put the welfare of a child first (regardless of negative feelings about the non-resident parent) or they don't. If you have the first type of ex you have a great chance at building good bonds which involve all adults over time, maybe with the odd blip. Otherwise you have the kind of ex which, for whatever reason, is so eaten up with emotion that they cannot put the welfare of their child first and are hellbent on making things as difficult as possible for the non-resident parent, whether they realise it or not.
I have friends who have great relationships with their stepchildren and their mothers and know others who, like us, have a nightmare of a situation. We do everything that has been said on this thread and more to try and build up bridges (after nearly 9 years together and we are also a 2hr drive away) and STILL the ex is using their children to fight this battle that she insists on maintaining. Now DH doesn't get to see his children. He has never hurt them, hurt her, been violent, taken drugs, committed a crime, made a bad parenting judgement or anything that a court may consider a court order for and as for money, well. So why is she still playing games, hurting her children in the process and continuing the drama? It all really kicked off after I got pregnant not long after we married a few years ago. Take from that what you will. This is a woman who can barely look at DH when speaking to him only to insist on seeing our baby in DH's car one time when he was collecting his children. Of course since that day his children have said mummy keeps telling them our baby is NOT their sibling. How horrible for all children involved that they couldn't relax with each other as other siblings do. She smirked when DH asked her about it. She knew the damage had been done and that was fine with her.
Feeding the situation with fuel keeps things going for somebody that could already be upset and needs some real help to come to terms with a situation. Perhaps everyone taking time to step back, calm down and reconsider the situation will help put things back on some kind of even keel in which to try and start a dialogue. DH's ex will only communicate via text too, with one worded replies, which strains communication and doesn't allow the children to see their parents being positive and civil towards each other. When trying to have phone conversation with them (which has stopped now due to their stress) his EX insists on having her ear to the phone at the same time which restricts his child feeling relaxed in the conversation.
I can understand how infuriating and unreasonable of her it must seem when she ignores your partner's concerns about his son only to then text and ask for money. However if your partner was to text back agreeing to give the money for the after school club then he could try, civily, to continue the texting conversation with asking after his son. If she still chooses to ignore those questions then it may be advisable to put his request in writing to her in the coming week as a precursor to taking a more formal approach.
If somebody refuses to communicate with a parent without valid reason in a parenting situation then, for the benefit of the child, you should follow through with a more formal procedure. If we knew 5 years ago what the situation would be like today we would have done it all different and not been so passive and agreeable. We behaved in that way because we're relatively nice people, love all of our children, don't want them witnessing ugliness and thought this is how divorced parents are MEANT to behave. However it has got us nowhere at all and because of it we both feel we have failed DH's children.
On a separate note congratulations on your pregnancy! Try to keep yourself mentally distanced from the situation at the moment. I know people share problems or issues in relationships because they are looking for support but maybe he could shoulder this one himself for a little while to allow you to enjoy your pregnancy. 