Do the laws of inheritance apply to the son mentioned in the question, or is it permissible for me to spend my wealth during my lifetime seeking Allah's pleasure, especially since this son squanders money in ways that displease Allah?

If a son doubts the faith, he does not remain Muslim. If the father ascertains this, he must strive to prevent him from inheriting, because a non-Muslim does not inherit from a Muslim by consensus. However, if the son is still Muslim, it is not permissible to prevent him from inheriting, as the impediments to inheritance are limited to difference in religion, murder, and slavery. If the father wishes to spend all his wealth during his lifetime on acts of righteousness, there is no objection. As for the hadith of Sa'd (may Allah be pleased with him), it pertains to bequests and charity during a death illness, and that it should not exceed one-third. If the father wishes to give gifts to his heirs, excluding the son who doubts the faith, there is no harm in that, especially if the son uses what he receives to disobey Allah.