He is considered a slot receiver right or wrong by most people in this league. He spent 53.5% of the time in the slot last season and 51.2% of the time in the slot in 2015. When you spend more than half the time in the slot, you are considered a slot receiver even if it is slightly more than half the time.
And he doesn't get paid what he deserves for a number of reasons. First, people believe he is a product of Brady and the Patriots' offense. Second, he is not an X receiver and X receivers are the ones who typically get paid because they are considered the big play makers. Third, he isn't a deep receiver. Most of his passes are short crossing patterns, screens, etc. and he picks up extra yards on the ground which typically does not give him a top of the league YPC (he had 11.3 YPC last year which wasn't even in the top 40 for WR).