You are sounding so very down. It's good you are looking for something to help you, taking time to take care of yourself and your needs. I hope you are recovering from your illness and also your son. Being unwell when you have toddlers around can be difficult.
People do tend to believe what they want to believe. It can be so frustrating. Your MIL will probably not listen to you and it seems doubtful if she is listening to her son. Some things are so painful that they are pushed away or pushed down to stop the hurt.
Ten weeks is a long time I know but in terms of your husband feeling better it is a short period. It would be great if he met you more often, went on a date, whatever, but his depression is not allowing him to think of anything or anyone. I know this is hard for you to understand. I found it hard for myself and I was the one with depression. I understand he talks to you and your son and probably sounds OK. The fact that he cannot stay for long suggests to me he is not ready to be part of the family.
Do you know what he does at his mom's home? Pretty much behaves as he does with you. A night out with the boys is different because he has no responsibilities and can talk or be quiet as he chooses. His friends will let him be whatever he does. You and your son want much more, and rightly so. You are after all his family.
I think I have suggested you talk to a psychologist, partly to have somewhere to talk frankly and get it all off your chest, but also to have help to understand why your husband behaves as he does. There really is nothing personal in his actions. By that I mean he is not doing anything to deliberately upset or confuse you.
It's good he has had a medication review and you think these new meds are helping him. It takes up to six weeks for meds to fully kick in although the improvement started weeks earlier with small changes. Possibly the previous meds had no effect at all. I have taken some of these. The period of time until the revised meds start taking effect is wasted in a sense.
Please be patient. I know you want your husband back and if he would return to his home, even as unwell as he is now, I am certain you would feel better. Keep the door open and when you see he is improving ask him to come home. Staying away too long may mean there will be no reconciliation.
I hope this is not all doom and gloom for you. I want you to know there is rarely any shortcut.