How can the son's desire for a modest wedding, in accordance with his religious commitment, be reconciled with his family's wish for a large wedding, given that he is the eldest son?

What we advise you to do is to hold the wedding ceremony as much as possible without extravagance or waste. There is no obedience to parents in holding a large wedding if it is beyond your capacity or reaches the degree of extravagance forbidden by Islamic law. Hasten to complete the marriage by holding a ceremony according to your capacity and ability, and be economical, for that brings blessing to the marriage. You should inform your parents and convince them of the impermissibility of extravagance. If you are unable to do so, then mediate through some relatives and righteous people to convince them. If they accept, then all is well; otherwise, there is no obedience to them in that matter.