Praise be to Allah.
Zakaah is due on trade goods, subject to two conditions:
1.That the minimum threshold (nisaab) has been reached;
2.That one full year has passed since it was acquired, because the Prophet (blessings and peace of Allah be upon him) said: “There is no zakaah on wealth until one year has passed (since it was acquired).” Narrated by al-Tirmidhi (632) and Ibn Maajah (1792); classed as saheeh by al-Albaani (may Allah have mercy on him).
Zakaah on trade goods must be paid on everything that is prepared for sale and dealing. That includes the medicines in the pharmacy and all other necessities that are sold there. As for things that are not for sale, no zakaah is due on them, such as furnishings, equipment (e.g. air conditioners and computers) and so on.
For more information please see the answer to question no. 50726.
Based on that, your brother has to evaluate the items that are sold in the pharmacy when one year has passed since he acquired the wealth with which he bought them, at the end of the year. Then he should add to that the profits made by the pharmacy that he still has, then he should pay zakaah on the total amount, at a rate of 2.5%.
It should be noted that the year in the case of trade goods starts from the time one took possession of the money with which those goods were bought, not from the date they were bought. For more information on that please see the answer to question no. 32715.
And Allah knows best.