Short answer. Half of it. Where an illegitimate child dies leaving no legitimate descendants, no surviving spouse and no children of his own, Article 903 reserves one-half of the hereditary estate as the parents' legitime. The other half is the free portion he could have willed away.
What the law says
The legitime of the parents who have an illegitimate child, when such child leaves neither legitimate descendants, nor a surviving spouse, nor illegitimate children, is one-half of the hereditary estate of such illegitimate child.
Civil Code, Article 903 — Legitime of the Parents. Read the full provision →
What the law says
If only legitimate or illegitimate children are left, the parents are not entitled to any legitime whatsoever.
Civil Code, Article 903 — Legitime of the Parents. Read the full provision →
The rule that answers your situation
Article 903 addresses exactly this case: the legitime of the parents who have an illegitimate child, when such child leaves neither legitimate descendants, nor a surviving spouse, nor illegitimate children, is one-half of the hereditary estate of such illegitimate child. A legitime is the portion the law reserves — it cannot be given away by will to anyone else, and a will that ignores it can be reduced. So on the facts you describe, one half of the net estate is yours by force of law. The remaining half is the free portion: if your son left a valid will disposing of it, that disposition stands; if he left none, it passes by intestate succession.
Where parents get nothing, and where they share
The article is equally clear about the situations in which parents are displaced. If only legitimate or illegitimate children are left, the parents are not entitled to any legitime whatsoever. Descendants exclude ascendants entirely — a surviving child of your son, legitimate or not, ends the parents' claim. Between those two poles sits a middle case: if only a widow or widower survives together with the parents, the parents' legitime is one-fourth of the estate and the surviving spouse's is also one-fourth. Establishing at the outset who actually survived — and obtaining the civil registry documents that prove it — is therefore the first step, because a single overlooked child changes the answer completely.
How this article reads today
The shares in Article 903 were worked out on the classification of children used when the Civil Code took effect in 1950, and they must now be read subject to the Family Code. The Family Code provides that the legitime of each illegitimate child is one-half of the legitime of a legitimate child, and that except for that modification all other provisions of the Civil Code governing successional rights remain in force. It also recognises a single class of illegitimate child, so the older sub-classifications the Civil Code used — natural, spurious and the rest — no longer apply. For the question you asked, the one-half parental legitime still stands; but if children of the deceased do exist, their shares are computed under the Family Code rule, not the 1950 categories.
What the article does not settle, and what to do
Article 903 fixes a reserved fraction. It does not tell you what the estate is worth, and the legitime is computed on the net estate — after debts, funeral and administration expenses — with certain lifetime donations added back. It does not deal with the estate tax, the extrajudicial settlement, or the transfer of titles, each of which has its own requirements and deadlines. It does not decide filiation either: where the relationship is disputed, the birth record and any acknowledgment become the whole case, so obtain certified copies from the civil registry early. And it does not apply where the child was legitimate, which is governed by different provisions. Gather the death certificate, birth records of every possible heir, the will if any, and a list of assets and debts, then take them to a lawyer before signing any settlement document.