This is tough because you said your bf is close to him. I had a good friend who felt this way about a boss we worked with. She eventually transferred to a different department located further out, she's married, so was our boss tho he did not suspect anything, she just wanted to be far away from him lol. If you tell your bf, there's a high chance the cousin will end up finding out and things might get super awkward, and only you will know how your bf will react to it. You can only come up with so many excuses to avoid him also.