Hi wishes,
Huge hugs to you, it's so hard supporting someone through a MH crisis. Its good that he has taken on board that he's not in a fit state to loom after your DS at the moment. If DS is happy and safe with MIL short term thats great, If you feel its safe for him to come home when you are arround then maybe you can work something out with MIL that will work for now. You are the only one who fully knows your situation and what is best for all 3 of you right now. When DH is poorly the stability of being at home with me is very important to his recovery so I can really understand you may not feel asking DH to leave till he's better is appropriate, and so long as you feel that you and DS are safe when he is at home with you, theres no need for him to leave either. Normality is likely to be what he needs right now. Also I don't see how sending him to MILs and keeping DS with you would help when you are at work either as I'm guessing this is the reason for DS going to MILs.
What support do you and DH have from MH services? Are they aware he is poorly? I know he is worried about his medical records impacting on his driving (my DH would be terrified if he thought he would lose his liscence too) but if he is in a MH crisis I have doubts about how appropriate going down the private councelling route will be. My DH suffers depression and psychosis and doesn't have insight into being ill when he gets poorly. It means sometimes I have to speak to his CPN behind his back, which I hate doing, but from experience with my DH his illness can quickly deteoriate to needing police detention so I have learned the hard way to step in sooner rather than later. Personally if you are worried he's a risk to himself I think you need to step in via his GP or CPN and get help urgently.
You need some support for you too and he doesn't get to tell you who that can be. I personally don't find my family helpful to support me when my DH is ill as they hold the stress I feel against him, and DHs family are not suppottive either. I have a CPN from his team for carer support who I can speak to again I've had to do this against his wishes in the past because I needed that support. I also have councelling available through work, wouldcthat be an option for you? Ultimately if its your Mum you need to talk to, then talk to her, even if its not what he wants, because its what you need.
Do your work know your situation? I'm lucky my boss is very supportive. You may be entitled to carer's leave, I get up to 3 days a year for emergencies, my boss has also let me take annual leave at short notice eg ringing up on the morning I need off. I think it would be more than reasonable for you to see your GP for your own needs too and consider asking for a sick note to take the pressure of work off you whilst you are getting things sorted with DH and your DS.
Most importantly put your own MH needs first, make time for yourself however bad things are. If you go under you can't look after DH or DS, so you need to be your priority however bad things are. Its like on a plane how they tell you to put your own oxygen on first.
Feel free to PM me I know how difficult it can get, and how easy it is for someone else MH problems to swallow you at times.