Is zakat obligatory on money that a father invested in building a commercial property, knowing that it has exceeded the nisab, and given that the company has started to lose money, thus decreasing his share? And is zakat obligatory for the current year only, or for all previous years, especially since he was unaware of the obligation of zakat on this money?

Zakat is one of the pillars of Islam and it is not permissible to delay it without a valid excuse. Your father sinned by delaying it, unless he was ignorant of the ruling. He must pay the zakat on his share of the property designated for trade for each year, unless it fell below the nisab (minimum threshold), in which case no zakat is due on it. If he is unaware of the amount due from him, he must exert his best effort and act according to his best judgment. He must pay it for all the years he delayed it, and he should act on his best judgment in estimating the wealth and the number of years if he is uncertain, based on the words of Allah Almighty: "So fear Allah as much as you are able."