Yes, he will HAVE to pay child maintenance. If he doesn't, the CSA can pursue payment, via an attachment of earnings order. They can also suspend his driving licence. They can also put him in jail. So, when doing your sums, count what you're due.
Your Council Tax will now reduce by 25%, as you are the sole adult occupier.
You may qualify for Tax Credits. Here is the calculator.
taxcredits.hmrc.gov.uk/Qualify/DIQHousehold.aspx
Child Benefit is paid to the mother also, so you'll get that.
Regarding your debts, you may qualify for an IVA (England) or Trust Deed (Scotland). Basically, all of your debts (not including mortgage) are added up to calculate your total debt. You work out how much you can pay per month and that amount is divided between all your creditors. This can be substantially less than what you would be paying monthly at the moment. After 3 years, any debt still outstanding is written off and you are debt free.
If you are struggling to pay the mortgage, obviously you could move to a cheaper house, but, if you don't want to do this, there is a Government scheme. Not sure what its called. CAB would help. I think you pay 25% of the interest only, for 2 years. By then you're expected to back on your feet.
Hope this helps! x