You will need to inform housing that he has come to live with you, that may either be housing benefit if you receive that, or Universal Credit where you can report a change on your journal. Both agencies will recalculate your entitlement.
There may a be a reduction in benefits if they expect him to make a claim and contribute to the household.
If you are struggling to cover the cost of essentials because of issues with your housing benefit/element, contact your local council to see if you are eligible for a Discretionary Housing Payment.