Is it permissible for me to advise my son to marry another woman because his current wife is unable to conceive, despite my fear of offending his wife?

There is no objection to encouraging a son to marry for the purpose of procreation; rather, he should be advised and guided towards it, as it is a legitimate purpose. The Prophet, peace and blessings be upon him, encouraged marrying a loving, fertile woman, saying: "Marry the loving, fertile woman, for I shall outnumber the other nations through you." The son should be encouraged to marry, and you should help him however you can. It is preferable for him to retain his first wife if she is of good character and religious, and then marry another loving, fertile woman in addition to her. If she does not accept polygyny, then there is no blame upon him for divorcing her.