I think he might have a block about interpreting his own feelings. Using the adjective 'boring' to describe most things he doesn't really rate is a bit simplistic. A series is about people, real life type situations, the human condition: presumably if others watch it enough for it to get air time, it must have some positive features. It would be more expressive of him to say the characterisation of a main person was clichéd, or the humour was rather trite. Or that he watches it when he just wants to flop after work, but wouldn't choose it on his day off, for binge watching.
Not enough to be a red flag, but it could be a bit socially inept, if you've been emoting about why a programme really engages you, and putting your heart into reliving an episode, and he says "Oh, it's boring". Part of life is being connected and impressed by the enthusiasms of another person - sometimes the appeal of a book or genre of music grows on you over time. If he isn't giving a new idea head space or a try, he maybe isn't open to the ideas or point of view of others. It's a little rude, as well to, dismiss something that another person has really gone over board on praising. More sensitive to say: " I'll have to give it another go, as you like it so much. Perhaps I've missed something. Sometimes things just stay in your head, when you've been feeling scathing watching them, and you like it much better on the second go."
Strong opinions can be not great, but you'll probably find out more as you go on. That's often the way, you let something go as just an idiosyncrasy, and then it happens over something where you feel they ought to have seen the other pov, a bit more, and they are not really open to spiritual growth and synthesizing new ways of thinking. Obviously TV programmes or films are not really a deal breaker.