Short answer. Not your brother — the legal (intestate) heir who holds the property in the interim is the one Article 885 requires to give sufficient security before taking possession, and only with your brother's own intervention. Once your brother's future date arrives, the property passes to him from that heir; the security duty falls on the interim possessor, not on him.
What the law says
But in the first case he shall not enter into possession of the property until after having given sufficient security, with the intervention of the instituted heir.
Civil Code, Article 885 — Designation of a Day or Time. Read the full provision →
Two kinds of future-dated institution
Article 885 opens by validating what your brother's will apparently does: the designation of the day or time when the effects of the institution of an heir shall commence or cease shall be valid. A testator may fix a future date for an heir's rights to begin (your brother's situation) or, separately, a future date for them to end. Both kinds of designation are allowed.
In the meantime, the legal heir holds the interim position
Before that fixed day arrives, someone has to stand in relation to the estate. The article provides: in both cases, the legal heir shall be considered as called to the succession until the arrival of the period or its expiration. "Legal heir" here means the intestate heir — the person who would inherit if there were no will at all — not your brother, who is the instituted heir named in the will.
That legal heir, not the instituted heir, needs the security
The next sentence, introduced by "but," qualifies what the legal heir may do in that interim role: but in the first case he shall not enter into possession of the property until after having given sufficient security, with the intervention of the instituted heir. The "he" continues from the sentence before it — the legal heir who is merely "called to the succession" still cannot actually take possession of the property without first posting security, and your brother, as the instituted heir, is given a say in that arrangement.
What this means for your brother
Because your brother is the instituted heir whose rights only commence on the future date, the security obligation in this article is not his to satisfy — it belongs to the legal heir who holds the property in the meantime, precisely to protect your brother's eventual right to receive it intact. Your brother's role in the interim is to intervene in that security arrangement, not to post the security himself; when his date arrives, the property passes to him from that secured legal heir.