What does the son mean when he tells his mother that she is already “wealthy”?

What the son means is that his mother already has everything she needs in life. More than this, her grateful attitude encompasses the whole of her existence. By modern standards, she has achieved what many consider impossible: contentment through a simple life that is rich in love and peace. So, this is why the son tells his mother that she is already wealthy.
In the story, Samuel is the son who speaks these words to his mother, Berlcha. He has come from America for a visit and brings money to renovate his parents' village, Lentshin. However, his parents tell him that the village will have no need of the money. This surprises Samuel very much, until he sees the coins his parents saved from cashing his money orders.
When Samuel asks Berl, his father, why they did not spend the money, the latter gives a surprising answer. He tells Samuel that their needs are provided by their garden, the cow, the goat, and the chickens. Also Berlcha sells chickens and eggs, and the profits are enough to purchase flour for bread. Both Berl and Berlcha rely on the land and their animals for all of their daily needs. In the winter evenings, Berl sits quietly while Berlcha spins flax at the spinning wheel.
Samuel returns to Lentshin after forty years, and he is determined to help his old village. However, he soon learns that his conception of poverty and wealth are based on faulty, preconceived notions. To his astonishment, he discovers that every single one of the villagers is happy with the simple life in the country. This is why he concludes that his mother is already wealthy and, therefore, has no need to pray for wealth.