"He wears a blue sweater."
The convention is that pronouns referring to myself can usually be dropped without ambiguity. It would cause confusion if pronouns referring to other people are dropped.
The watashi is implied because I am the one speaking to you. Both you and i are inherent in whatever i say. However, if I want to say something about him, i have to be explicit.
来ます (きます/kimasu) means "to come," yes, but 着ます (きます/kimasu) means "to wear." this is where duolingo's lack of kanji can trip you up.
on the difference between 着ます and 履きます (はきます/hakimasu): 着ます is used when talking about clothes that hang from your shoulders, i.e. things you wear on your upper body, like shirts or dresses; 履きます refers to clothes you wear on your lower body, like pants. Japanese actually has kind of a lot of different verbs to describe wearing different types of things on different areas of the body.