Hi Lisa, firstly can I say how sorry I am for what has happened. What a shitty thing for your DH to do on NYD.
You need to seriously consider going to see a lawyer that specialises in family law. CAB are not always that helpful and can sometimes give bad advice as you have to bear in mind that the majority of the people who work there are not legally qualified. They should however be able to point you in the direction of a family lawyer or else simply look in your yellow pages. If possible, try to find a lawyer with a public funding (legal aid) franchise. These are few and far between unfortunately but would be best to find one as you may well be elgible for public funding (legal aid) depending on the available capital assets you and DH have. I would also recommend finding a lawyer who is a member of Resolution (look out for this on their adverts) as these lawyers tend to be able to reach a more amicable agreement than non-resolution members.
It is still very early days and you may therefore not be thinking about divorce/legal separation but you need to know where you stand financially.
Your DH is legally obliged to support you and DS and has been so far. Clearly things will be tighter financially once he is in rented accommodation but he can't simply turn the tap off and stop paying you.
In the short term, speak to your mortgage lender to see if you can take a payment holiday or perhaps re-mortgage on more favourable terms. In the long term, you need to consider whether it is viable for you to remain in the former matrimonial home (FMH) If it is a modest house, ie a 2 bedroom which is the minimum you require for you and DS then it seems pointless selling and you should be able to stay in the FMH although obviously you will need to come to some sort of arrangement with your DH about how/when he could realise his share of the equity.
If however the house is larger than your requirements, ie a 3/4 bed house then your DH could potentially argue that you are 'overhoused' and that the FMH should be sold and a smaller/cheaper property purchased for you. Again, some sort of agreement would need to be reached with regards to your DH's share of the equity and any other matrimonial assets. It is quite common (if viable) for the wife to remain in the FMH (or a cheaper property if sold) and for the H to receive his share at a later date, the usual triggers being the child reaching 16/17/ceasing full time education, the wife's re-marriage (in some areas some Judges will also say cohabitation) or the wife's death or voluntary sale of the property. (or you could buy him out if possible) It is not automatically a 50/50 division of the equity. Lots of other factors need to be taken into account, to include any other assets and your DH's pension provision if any. Sometimes this can be a significant matrimonial asset.
With regards to maintenance, he is legally obliged to support your DS. The Courts no longer have the jurisdication to make child maintenance Orders although they can approve any financial agreement reached which incorporates child maintenance. The CSA previously dealt with child maintenance and for 1 child the rate was 10% for one child (of your DH's net income, defined as gross income less tax, NI and pension contributions) Last November the CSA was replaced by a new governing body and I think the assessment rates are similar but don't quote me on this.
As paranoid says, make a thorough list of your requirements, ie mortgage, council tax, utilities, food & housekeeping, clothes and other things like prescriptions, dental, opticians, clothing and other non regular outgoings that still add up over the year.
If things are amicable between you and DH it may be worth you attending mediation in an attempt to resolve outstanding issues. Mediation can sometimes assist parties reaching an agreement on matters such as financial support and issues relating to children, ie residence and contact.
I would recommend contacting Resolution who can help you find a specialist family mediator and perhaps a family lawyer who will hopefully help you reach an amicable settlement.
Good luck