Is it better for him to do Hajj on behalf of his parents or to contribute to the building of a mosque?

Praise be to Allah.

If there is an urgent need to build or renovate the mosque, then he should donate the costs of Hajj to the building of the mosque, because of the great and ongoing benefit of doing that, and because it will be helping the Muslims to establish prayer in congregation.

But if there is no urgent need to spend the cost of a voluntary Hajj on renovating the mosque, because there are other people who can contribute to the renovations, then doing Hajj voluntarily on behalf of his parents, either himself or through a trustworthy proxy, is better, in sha Allah, but it cannot be done on behalf of both of them in a single Hajj; rather he should do Hajj on behalf of each of them separately. End quote.

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).

Majmoo‘ Fataawa Ibn Baaz (16/372).