Why are you assuming his isn't? High utilization on one card =/= bad credit

>From what I read I have my doubts, because your credit score is great and his -apparently- isn’t.

OP being added as AU won't help the father's score at all.

>If you signed something, check the contract.

AUs don't need to sign anything. Many lenders don't even take AUs to be of age where they legally could sign anything.