Hope you kicked him out now! Sorry this is happening to you OP but it WILL get better - good luck and don't feel obliged to do anything for him.
With bills, make sure both your names are on the account (not just yours), therefore he is partially responsible for paying these.
With mortgage, he is partially responsible to pay, but he can choose not to. Make sure you don't have ANY joint accounts with money in, he has every right to just take savings, money etc in both your names - so either close them or take the money yourself (let a solicitor sort who gets what but you can just take it).
Send him an immediate request for child maintenance (use gov website to ask for how much) and tell him when and how to pay. If he refuses, go straight to CMS without warning to him. If he moves in with her the other kids will be taken into account but you'll get something.
Engage a decent solicitor now, no need to hesitate. They will do everything for you (but don't waste money on asking for washing machines and the like, use them for the big issues like entitlement to pensions, savings, expensive items etc.) and house.
Change your access passwords to your phone, email etc.
And if he comes crawling back, say no!