All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
Most scholars held that it is obligatory on the Muslim (who is able to perform Hajj) to hasten to perform Hajj without delay. Please, refer to fataawa 115756 and 91223. Hence, it is impermissible for your son to delay performing Hajj as long as he is able to perform it. He cannot give priority to financing his father's Hajj over performing Hajj himself because his father is not obliged to perform Hajj since he does not have the means for it, and the son is not obliged to pay for the expenses of the father's Hajj in this case.
If the son is able to perform Hajj for himself and also pay for the costs of his father's Hajj, then that would constitute an act of kindness and dutifulness towards his father by helping him perform the obligatory Hajj. However, if the son is unable to finance both his Hajj and that of his father, then he cannot delay his own Hajj in order to finance his father's Hajj, as has been stated above, because he bears a sin for delaying performing Hajj for no valid reason. Hajj is an immediate obligation; whoever is able to perform Hajj must do so right away.
Allaah knows best.