Ffs Lena, give your head a wobble. If you have sent ds to school today, collect him early and keep him off school until a residence order is in place.
To some extent you still appear to be a rabbit caught in the headlights, which isn't surprising given your ex's onslaughts over the last couple of days.
You need to wake up and be aware that in order to keep your ds safe you will have to channel your inner Boudicca and invoke the spirit of Machiavelli in order to outwit your ex.
At the time of writing there would seem to be nothing to stop your ex removing your ds from school at whatever time he chooses and putting the child on a plane with MIL later today in order to 'protect' him from the marital fallout.
Your ex will then claim that he was acting in the child's best interests as he was becoming increasingly distressed by YOUR unreasonable behaviour, and he will no doubt find some tinpot psychologist practising in MIL's location to prove his point.
I'm not saying he'll do this, but I would put money on him giving consideration to such a plan because it is abundantly clear that he will stop at nothing to get back at, or hurt, you.
As a loving and caring mother, your child's welfare and well being are paramount and you are determined not to stand in the way of your ds having a loving relationship with his df.
However. throughout the course of the last few days your ex has treated your ds as if he's a possession and now he's turned him into a prize that he's determined to win.
Your son is understandably confused, and most probably frightened, by this recent turn of events and you must do your utmost to undo the harm his df has caused him - and this means keeping him away from your increasingly unhinged ex until such time as court orders are in place.
As I said upthread, possession is 9/10ths of the law. Your child is NOT a possession but, in order to keep him safe, you must retain custody of him at all costs and do everything in your power to prevent him having any contact with his df until such time as you are both protected in law.
I hope to god that your current solicitor is treating your case as a matter of the utmost urgency and that interim orders are in place by the end of the week.
Is there any way you can fund an application for residency without relying on legal aid?