Which is more befitting: Hajj, or building a house for one's son, knowing the son and his wife are in straitened circumstances, the long duration of saving money for building, and old age?

A father's financial support for his adult son is obligatory if the son is unable to provide for himself, and housing is part of the obligatory expenditure for children. Among the conditions for the obligation of Hajj is that one's wealth must exceed what the person and their family need for housing.

If you can afford to rent accommodation alongside Hajj, then Hajj becomes obligatory for you. Building a special house for the son is not obligatory, as the need for housing can be met by renting.

However, if you build him a house with the money you have before the Hajj season, there is no harm in that. But if the Hajj season arrives and you possess the rent for accommodation, then Hajj becomes obligatory for you.

If the dwelling is spacious and exceeds one's needs, or if it is valuable and surpasses similar properties, then selling the surplus or exchanging it for a more modest dwelling to cover Hajj expenses is obligatory according to the Malikis, Shafi'is, and Hanbalis, but not obligatory according to the Hanafis.