I know it doesn't seem like it but this can all be handled. I'm not saying I have any answers however... all I can say it's a constant ball-ache to have to deal with unreasonable controlling people, particularly when there's a vulnerable small person in between you.
The thing is, by withholding contact you are immediately giving him some ammunition; something to hold onto and hold against you. I can tell you from my own experience that unless something has happened to make you KNOW that it's 100% not in your child's best interest to see this person, that when you withhold contact the pressure is just unbearable, as time goes on.
Forgive me for not knowing all the details, I have read and lurked so know some but not all. If you are in court on Thursday then will you attempt to raise this issue there? I know it's not a children hearing but I know from a friend's experience that it can be discussed. And in my own case, we are back in court in a couple of weeks for the finances but are also hoping to discuss my ex's attempt to get a variation in our order.
I really do get it, I do, honest! I had a non-mol out against my ex at one point, and he used the wording in that non-mol to stop contact completely, citing the fact that it said "not even indirect contact is allowed" - we were away at the time and he took that to mean that he couldn't Skype the children so overnight just disappeared from view. He has also made numerous false allegations against me (claiming that I had threatened suicide; I have never done any such thing) which led to SS involvement. Thankfully my experience of all these agencies has reinforced my belief that my ex is an utter loon and they have backed me completely. It's a huge relief but still has been unbelievably stressful.
Playing devil's advocate with what you've just said, I would say - okay, so forget the cheque - is contact in your DS's interest or not? Because breaking things down a little - yes, he's being an arse by sending it to MILs, but at this stage you don't know that it will bounce. So it does look like you are angry about his stupid behaviour regarding the cheque and punishing him by withholding contact with DS.
What do you mean about the TAS? Anyone without a court order has the ability to withhold contact, are you saying that if you choose to stop contact then they will back you up, or it will be deemed as acceptable for you to have done so? (Apols for my ignorance; I haven't heard of that service before...)