It seems to me that what you need most of all right now is a bit of time out.
Your husband has been an idiot - studies are not an excuse for shutting down a relationship or putting it on hold. The fact you were open for an affair shows that you were so desperately in need of love and attention (I hope the OM is a decent guy!). A long term furture with him isnt impossible but it may be not very likely too = the sheer stress of divorcing is pretty awful, and not all relationships survive it.
at the moment you are in a really really high stress situation and maybe you can't see the wood for the trees (maybe you can, I dunno; but it's clear you're absolutely stressed out).
Just a suggestion - is there anyone you and DS can go and stay with for a couple of weeks? he can't be in school yet, I'm guessing. Maybe use the excuse of 'giving your husband more uninterrupted time to study'. If you do have a friend you can stay with, maybe a bit of distance away, that might give you breathing space and time to decide exactly what you -do- want.
It also gets you away from your mother for a bit,
Then you can decide if you want to give head-in-the-sand husband a last chance. (can't help feeling that ostriches don't often change their habits, but now and then they do. ). You can also decide how much you -really- like OM, or if its just that he's giving you some of the attention and love you need ... and how interested he would be in a LTR, or how good he'd be.
After that, you can make some decisions. Without money or a place to go, the truth is that you might end up in a really difficult situation - and having to rely on your mum sounds a nightmare. Also you'll need to plan how and what to tell your DS to help him handle it best.
I hope I\m not too cautious here, but a bit of clear space to think in often helps people clear their minds. And if you do decide you don't want to give husband another chance, then you need to have planned, really planned, what you will do and how you will support yourself. Otherwise you could end up in a worse situation (worst case; which is better, being reliant on your husband's income or your mother's, or maybe having to move in with her?)
Deciding what you =really= want and then the practicalities of how to get it will help a lot.