What is the ruling of the Sharia on a gift that was not fully possessed, and was revoked due to moral duress? Is it valid or not?

A gift is not binding by mere utterance, according to the majority of scholars. It only becomes effective upon possession. The donor has the right to retract it before possession, and it remains his property to dispose of as he wishes.

Therefore, if your father retracted the gift to his cousin's son before he took possession of it, his retraction is permissible, and he is not obliged to give anything except by his own good will.

However, if your father retracted the gift after his cousin's son had already taken possession of it, then the gift is effective and cannot be retracted, as long as the donor is an adult, of sound mind, and acting of his own free will.