彼の視力は衰えつつある。

he; him; his; boyfriend

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

eyesight

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

to become weak; to decline; to wear; to abate; to decay; to wither; to waste away

while; even though; despite

to be (usu. of inanimate objects); to exist; to live; to have; to be located; to be equipped with; to happen; to come about