If the university wants a release signed by your son, then you have your son sign the release.
The university cannot release the academic records of an adult student to another person without the student's consent (or court order).
If you and your husband are living together and are the legal parents of the son, you have no legal duty to pay for his college education. If your son does not sign the appropriate documents, you stop paying his bills from the university.
The university likely has blank forms at its website for releases and requests for transcript.
Many universities also have online access to students' records. You can have your son sit in front of a computer with you and sign into his account and show you his grades.
If your son is lying about his grades, you likely should demand to see computer screens in real time or get the transcript from the university sent directly to you. This would reduce the chance that the information is altered.
You simply instruct your son to provide you (in a reliable and verifiable manner) what you need. If he doesn't or won't, you realize that there is a reason for that and cut off the $$$.
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