Short answer. Your son can use 'Junior' — Article 375 of the Civil Code reserves that designation for a son who shares his father's full name. Roman numerals (II, III, and so on) apply to grandsons and further descendants with the same name, not to sons.
What the law says
In case of identity of names and surnames between ascendants and descendants, the word "Junior" can be used only by a son. Grandsons and other direct male descendants shall either: (1) Add a middle name or the mother's surname, or (2) Add the Roman numerals II, III, and so on.
Civil Code, Article 375 — Junior, II, III. Read the full provision →
The one-generation rule for 'Junior'
Article 375 draws a precise line. 'Junior' is reserved for a son — the first-generation male descendant who shares his father's full name. If your name is Juan dela Cruz and your son is also Juan dela Cruz, he is Juan dela Cruz Jr. That is the only relationship where 'Junior' applies under the Civil Code. The rule makes sense as a naming convention: 'Junior' signals one generation removed, and the law ties it to exactly that distance.
What grandsons and further descendants must use instead
When the same name carries into the next generation — the grandson who is also a Juan dela Cruz — 'Junior' is no longer available. Article 375 gives grandsons and other direct male descendants two options. They may add a middle name or the mother's surname to distinguish themselves. Alternatively, they may use Roman numerals: II for the grandson, III for the great-grandson, and so on. The choice between these options belongs to the family, but 'Junior' is closed to them by law.
Why the distinction matters in records
The distinction matters most in official documents. Birth certificates, civil registry entries, government IDs, and legal instruments all carry the registered name. A grandson registered as 'Junior' technically carries a designation the law does not authorise for his relationship to the elder bearer of that name. While civil registry practice may tolerate variations in informal use, documents that must match the registered name — passports, land titles, bank accounts — will follow whatever was entered at birth registration. If a family member was registered with the wrong suffix, a correction proceeding before the civil registrar may be required.
Practical guidance when naming a child with the same name
When registering a newborn son with the same full name as his father, 'Junior' is the appropriate suffix and the civil registrar will recognise it. When registering a grandson with the same full name as his grandfather, the correct options are a middle name, the mother's surname, or a Roman numeral. The numeral II is most commonly used for the grandson generation. Families who want to carry the name further would use III for the next generation, though this situation is uncommon enough that each case may benefit from a check with the local civil registrar to confirm how the entry will be recorded.