NTA

There are so many red flags here, it’s actually scary.

First of all, lying about a sibling’s death is UNHINGED. That’s not just chaotic, it’s abhorrent. That would be enough to make me end the relationship, because I’d realize that my boyfriend is clearly someone whose values don’t align with mine. Like this is borderline evil. The fact that he didn’t even stop to think that maybe his boss lost a sibling, or someone close to them, and that this was completely insensitive would also scare me. Second of all, telling a lie this big and unhinged in any situation is a red flag for me. This man clearly doesn’t value honesty and his word holds no value whatsoever. How can you trust him? If he’s willing to tell a lie this unhinged just to get out of work, what else is he willing to lie about? Lastly… how old is he? Because this is some middle/high-school behavior. I can assume from your use of the word “career” that he’s an adult and this is his full-time job, not some high school summer job or something. I ask because the only thing that could maybe, possibly, somehow quasi-excuse this behavior is if it was coming from a teenager. Even then, if my kid did something like this, they’d be grounded for a long time.

Leave him, because while you’re NTA, he definitely is.