What is the ruling of Islamic law on concealing impaired hearing from a suitor before marriage, and is it obligatory to inform him of it?

Weak hearing in one ear is not a contagious defect, nor is it repulsive or an impediment to enjoyment. Therefore, you are not obligated to inform the suitor about it, and there is no sin upon you for keeping it a secret. However, it is preferable and better to inform him so that his future knowledge of it does not become a cause for the deterioration of the situation.