Is it more befitting to help one's son who is about to get married with money, or to perform Umrah with this money, knowing that I have performed Umrah several times and will not be able to help my son if I go for Umrah?

If you have previously performed Umrah, it is better to help your son get married, due to the far-reaching benefit it provides and the protection it offers him and his future wife from committing forbidden acts. The Islamic legal principle prioritizes acts of worship, so that which has a far-reaching benefit is preferred over that whose benefit is limited, unless the limited benefit is one of the obligations of Islam. This prioritization applies if the son is capable of marriage and needs assistance. However, if he is unable to marry and needs chastity, and his father is able to marry him off, then it is obligatory for the father to marry him off.