Interested in Language
He was educated at Eton, where he made his first intimate friends-Richard West; Thorns Ashton; and Horace Walpole, the son of the prime minister. After a little over four years at Cambridge he left without a degree to make the grand tour of France and Italy as the guest of his friend Walpole. The death of West in 1742 was a depressing event for Gray, who, having quarreled with Walpole, felt keenly the loss of this gifted and congenial friend..
Source: The Norton Anthology of English Literature, Thomas Gray (1716-1771)
The blue word (the son) refers to who? If it refers to the four friends then why itís not written the sons? (I mean plural)
Many thanks in advance.