en_tn_lite_do_not_use/mrk/12/37.md

calls him 'Lord,'

Here the word "him" refers to the Christ.

so how can the Christ be David's son?

This can be written as a statement. Alternate translation: "so consider how the Christ can be a descendant of David" (See: rc://en/ta/man/jit/figs-rquestion)