Depends: if he’s a good person and you just don’t feel chemistry then tell him! Likely he’ll be sad, but will move on like all break-ups. If it’s because of an issue you’ve discussed already and he’s not getting better, or if it’s a deal-breaker that he knew about (or that common sense says, like cheating), just tell him ‘we’re done’ and why, and walk. After that you owe them very little.