Open a case with the child maintenance service, they will assess him and tell him what he should be paying. He will get a reduction because there is a child living with him but you should get more than £10. If he still refuses to pay they will deduct the money straight from his wages.
Are there any school uniform grants you can apply for? I know in my area you can get about £140 if you qualify and if you’re short of food is there a food bank you could use to help you out.