Are ethical laws merely social conventions, with reference to kant.

The simple answer to this question is no.  For Kant morality is
possible because human reason provides access to the necessary
conditions of human understanding.  These necessary conditions are not
conventions in the sense of being true only contingently, or based on
intersubjective agreement; rather, the conditions of morality are a
reflection of the human condition vis a vis the requirements for our
understanding.  Of course, most contemporary followers of Kant,
notably John Rawls, offer a version of ethical reality based on a
social construction.

If Immanuel can't, then noboy can.