A character who is outwardly grumpy, but inwardly mild, forlorn or remorseful (or something else entirely) sounds compelling. There’s a disconnect between his inner experience and his outer appearance and that makes me wonder: What’s happening for this character? What’s getting lost, modified or missed in translation?
That being said, you write that he’s always shouting or saying something unkind. And that makes me wonder about the development of this character. Without having seen your manuscript, I can’t say for sure whether this is warranted or not, but let me give you some points to consider.
Has this character’s dialogue gone over the top? In other words, is his dialogue creating a character or a caricature? If you stripped away the narrative, does he sound like a red-faced cartoon mouse or a human being? Even the grumpiest of grumps should be capable of a civil, if abrupt, routine exchange at the check-out counter. You might not have such an exchange in your novel, but the reader should understand he’s capable of one.
Are you ignoring the use and grace of subtlety? If your character has a raised voice or nasty comment every time he opens his mouth, I wonder if you’re missing some opportunities to reveal the more subtle aspects of his nature. He might not be pleasant in an exchange, but he might be curt to someone’s warmth, or dismissive of another character’s vulnerability. Sometimes, these behaviors can do more damage than a lot of bluster.
Lastly, challenge yourself to write a scene in which this character goes against his grain. What would happen if he had, for a change, a kind word for someone? Or if he whispered instead of yelled? Do this in a moment where it’s least expected. This kind of experimentation can be revealing. You may find that these scenes just don’t work with your character as you’ve created him. But you may, in fact, find that they add depth or take the scene to an unexpected—but apt—place.
Don’t let your preconceived notions about your character get in the way of fully developing him. That may not be happening in your novel, but it’s a good idea to take a closer look to be sure.