Short answer. He must restate the dispositions. Article 835 provides that a testator cannot republish a will void as to its form without reproducing its dispositions in a subsequent will. A mere reference to the old, defective will is not enough; the dispositions themselves must actually be reproduced in the new one.
What the law says
The testator cannot republish, without reproducing in a subsequent will, the dispositions contained in a previous one which is void as to its form.
Civil Code, Article 835 — Republishing a Void Will. Read the full provision →
Reference alone is not enough
Article 835 answers this precisely: the testator cannot republish, without reproducing in a subsequent will, the dispositions contained in a previous one which is void as to its form. Since your father's earlier will was void because of a defect in its form, he cannot simply write a new will that says something like 'I hereby republish my earlier will' and expect the old dispositions to spring back to life. The law requires the actual dispositions themselves to be reproduced in the new will.
Why a void-as-to-form will cannot simply be incorporated by reference
A will that is void for defective form never validly existed as a testamentary instrument in the eyes of the law, even though it may express exactly what your father wanted at the time. Because it never had legal effect, there is nothing valid for a later will to point back to and revive merely by reference. The only way for those wishes to actually take legal effect is for your father to state them again, properly, in a new will that itself satisfies all the formal requirements the law demands.
What 'reproducing' the dispositions actually requires
Reproducing the dispositions means restating in the new will what property goes to whom and under what conditions, essentially rewriting the substantive content of the old will's provisions rather than gesturing at them. This can be tedious if the earlier will was detailed, but there is no shortcut the law recognizes; a will void for defective form contributes nothing on its own, no matter how clearly it shows what the testator wanted, and every disposition your father wishes to carry forward needs to appear again in the new document.
What your father should do going forward
If your father wants the dispositions from the earlier, defective will to actually take effect, he needs to execute an entirely new will that both satisfies the formal requirements the earlier one lacked and restates the specific dispositions he intends. Simply cross-referencing the old document, or attaching it to a new will without reproducing its contents, will not accomplish what he is trying to do under Article 835.