If the calls are upsetting you absolutely need to revisit this.
I'm particularly concerned about her raising expectations with her son that cannot be realised.
So as a start point I would stop real time calls for now. I'd replace this with a monthly email from mum that your partner can read and "edit" if necessary. Monthly because it's not fair to the child to live on a weekly rollercoaster of emotions.
He should in fairness articulate to his ex what he will "edit". For example - dad will bring you to visit or we will live together when I get home. Basically promises she can't deliver.
I would not stop contact altogether. Long term this could work against your partner and I'd be concerned about how you could reintroduce contact without distress. Having a relationship with both parents is generally regarded as optimal, but I think you have very good reason to dial it down significantly.
However your partner reading a monthly email from mum whilst they have a reassuring cuddle is very different to a real time video call every week and far less emotive for child than seeing a parent they can't touch/hug that they miss over a screen.
This also has the benefit that you have a record of the communication- something you might need in the future when she returns which demonstrates the "quality" of her parenting if you end up in court.
As pp's have already said, pull in support from the school and make them aware of the situation.
You might also want you consider speaking to children's social services. People are often scared about doing this, but they can absolutely signpost you to support and potentially even funding/referrals for counselling.
Again if you have a proactive positive case history with social services this will work for you in the future - especially if they are privy to and can case note her actions.