Is this birthday request coming from DSS or is this his mum putting words in the boys ear? That comment about your work is not something an 8yo would say/have any concept of.
My OH DD is reported to say an AWFUL lot, like the fact that she doesn't want to go on holiday abroad where she's been with her dad and families over the years, like she only wants to go for a week, like she doesn't want to spend time with me/OH, her and my DS.
ALL of it is a complete fabrication. The ex is a fantasist and we regularly get missives about the telly we watch or the activities we do not being 'girly' enough' It's all a load of utter bollocks.
I worry about the meet up. either he's complicit (huge problem) or he's totally naive and doesn't realise that the Ex is really causing massive problems here on purpose and he is being played like a violin, which is also a problem, as you are going to lose respect for him PDQ.
I am leaning towards thinking that you might have a DH problem first and foremost.
A decent H would be the first to say 'DS, of course Busymum is not leaving her bed so you can get in' He would either say to DSS to go back to bed or he'd go and be with him. at the least, he'd do this to not disturb your sleep.
He would actively discuss lying and biting and suggest an apology is owed to your DS
WRT the party, his mother can arrange a party for DSS and he can invite who he likes, you and your family do something separate, but you and DS are to be included. there is engineering going on here and it's not pretty. Time to call a halt to it now. take back control.
We also tend to do 'fun stuff' when OH DD is down, but that is mostly because it's more fun with us all there and the 2 kids get on really well and it's great to share. we have done some things as a 3 and then gone and done them again when DD is down. Perhaps that is down to you to push a little more? Establish some fun stuff when DSS isn't there?