Read those excellent sources posted below. I’ll pick it up from the Catholic perspective though.
As a rule, for a Roman Catholic like your son to marry validly, he must marry in the Catholic Church. Such a marriage in the Roman Catholic Church requires the active assistance of an authorized priest or deacon who asks for the parties to exchange consent and who receives it on behalf of the Church. It also requires two witnesses. This canon 1108 is called the “canonical form of marriage” for Roman rite Catholics. (Catholics of the Eastern Churches are bound to a somewhat different form of the celebration in which the blessing of the priest is emphasized.)
However, canon 1127 is an exception to this. If your son married an Orthodox woman in a sacred rite according to the laws of the particular Orthodox Church with the customary wedding blessing of its priest, the Catholic Church would recognize it as valid, and a sacrament — but unlawful. I urge them not to do that.
Even so, canon 1108 is an ecclesiastical (Church made) rather than divine law. So, the Catholic bishop, for a just cause, can dispense from the canonical form of marriage which is required of your Catholic son for a lawful marriage.
A dispensation is an act of competent Church authority that relaxes a Church law. This would allow a valid and lawful wedding before the Orthodox Church priest. However he must conduct the sacred rite according to the laws of that particular Orthodox Church and give the customary wedding blessing to the couple.
Assuming both your son and his fiancee are otherwise free to marry, either a Catholic or an Orthodox wedding with the dispensation could be permissible from the Catholic perspective.
Be aware that the Orthodox Churches have a different outlook on freedom to marry. They may allow people who have been married before and then divorced, to marry again.
For a Catholic or someone who wants to marry a Catholic, the Church would require the annulment process or something similar by the Catholic Church to determine such freedom. It does not accept the Orthodox equivalent of a Catholic annulment.