You may be able to claim Child Tax Credit if you’re responsible for children either:
You do not need to be working to claim Child Tax Credit. Only one household can get Child Tax Credit for a child so if his parents are claiming for him, then you wouldnt be able to.
It may be worth trying the benefits calculator here, but it would be useful to know what anyone else is claiming for him too. You could also look into Child Benefit if he is moving in with you permanently.