i would get your DH to contact CS and let them know that your DSS has decided to live with you now and see what they say. i dont know how you go with his mum getting the centrelink payment, but thats up to her letting them know otherwise if she doesnt and they find out she is required to repay that amount payed to her since DSS has moved in with you and she didnt notify them ITMS.

maybe a new custody agreement is a good idea too. not sure again how you would go about that

good luck and i hope you can work it out!