First, I am so sorry you are going through this. I once posted something pretty similiar on this board. My AH was doing drugs in the garage (male addicts love their garages)...and I wanted him out of my house and my life. He said he didn't have to go because he had every legal right to be there. Hard to fight that.
If you really want to leave and escape, you could do just what Elana suggested and leave him in the house and get your own apt. I don't know if you have a lot of money to lose/gain. For me, when I did own a house, I owed as much as the house was worth. But if you have a lot of money coming to you if the house is sold, it would make it harder to do that.
The possibility of going to stay at your mom's who will help take of your son and give you support is an amazing offer. I can only imagine how much that will help you. I have 2 kids so I know all about how hard it is to leave with kids involved.
I will say this to you: Beware of your trust in your AH never doing or using infront of your son. I believed 100% the same thing and would have gone to battle in his defense of this. Others warned me I was being naive if I believed an addict was capable of making good decisions. But I thought, their experience is so different than mine because my AH would NEVER do that. But he got worse as his disease progressed and he picked them up from school high and intoxicated. And a few days later (I wasn't informed about his picking up the kids wasted for 4 days) he put them in the backyard where he felt they were safe because there is a fence and went to get beer. He left them alone in the backyard for approx. 15 minutes while he was at the store. There is a busy road right outside of the gate to the fence...they can open the gate and a little creek that they could have drown in. It is still hard for me to believe he did that but he did. From then on it was supervised visits only. He hated me, HATED me immensely for me not letting him have the kids unsupervised and spit fire out of his mouth when he saw me. But now that he is clean (I think he is clean anyway), he thanks me for it and is disgusted by his actions. I am SO grateful and lucky they were ok.
So...what should you do? Protect yourself and your children. Don't think you can trust him to do the right thing...he isn't in control, his drugs are. And do whatever you can do that will make you sane. If you can't live another day with him, pack and go. Sanity is more important than a credit rating. But if it is more than you can bear to have to do that, then do whatever you have to do to get that house sold.
I was told that you could get a RO to protect your children from him is he is using but I don't know if this is reallly true since the advice was given from someone who was not a lawyer or law savvy necessarily...but I would contact the court/police in your area and ask questions. You can do get the order without a lawyer I am pretty sure.
I don't know if you are willing to do this and maybe this will get some disapproval from some, but if you knew for sure he had drugs on him and reported that to the police when he got in his car and reported his route and other info...he would get arrested for drugs and then it seems very plausible to have a RO for that reason. This takes real courage and is a last resort kind of solution. It was something I could never do but it was recommended to me by more than one person, including his mother at the time. If I could go backwards, I wish I had done it. Again, I am not saying this is what you should do...but it is an option...maybe not fitting for your situation..only you know that.
Your job is to take care of you and your son. The fact you have an illness only makes his behavior more crappy. He is being so disrespectful to you by not leaving and causing you more stress...you don't need that! If getting to your mom's is the right decision for your health and your child's well being...go. You can't put a price on health and well being and sanity.
Good luck. It is not an easy situation for sure.