Son turned 18 on oct 29 but premium won't apply my dependent tax credit. I have filled out his income from his part time job. Why is it not allowing me the credit?

Are you talking about the eligible dependant credit? If so, his income is too high. His net income must be less that your basic personal amount for you to be able to claim the credit. The basic personal amount is $14,398 for the 2022 taxation year.