WIBTA if I told someone the truth about why they were being excluded from group activities?

A few years ago, I met my partner's cousin (on his dad's side) for the first time when he came to visit our city (we live on different coasts). I quickly realized this was the type of person I would never hang out with normally, but I was cordial. My partner acknowledged, "Yeah, Fred's a lot of fun but he can be a bit much."

About a year ago we moved to my partner's hometown and have been spending time with his family here (all on his mother's side) and his childhood friends. Coincidentally, his cousin moved here also for work and the family welcomed him warmly, eager to spend time with this cousin they didn't know very well. At first he would be invited to all the things, but over time I noticed he was left out more and more. My partner would ask, "Hey, where's Fred?" and people would groan and make faces. Turns out that I'm not the only one that doesn't like being around him. He was still invited to Thanksgivings and big events like weddings, but casual family get-togethers like a game night or Sunday brunch, people have left him out. And this is a family that does a lot together.

He has noticed and actually messaged me asking if I knew why the family isn't including him in things. I guess he sees me as another "outsider" so feels a connection. WIBTA if I told him the truth? Because the truth is that most people don't like his personality. He's loud, abrasive, pushy, and inconsiderate. If you can him out on any of that, he will just say he's "fierce" and people need to learn how to deal with it. He changes the energy of any event he gets invited to. When my partner said he could be "fun" it's because he will be the one loud one at bar or club, trying to push people to dance, do shots (or drugs), or do karaoke, etc. He will loudly criticize you for ordering a menu item or drink that he disapproves of He doesn't respect other people's preferences, and like myself, my partner's family is mainly introverted. We don't need to "come out of our shell" and we know what we like or don't like. We're not kids (all the adults are in their 40s). An example of when we all went to a museum with grandma and the kids. There was a very cool sculpture that had a rope and a sign around it to keep people from getting too close. He wanted a group selfie and insisted we duck under the rope to get it. None of us were comfortable with that but he kept insisting. A few of us started to walk away and he called us p**sies. And then he laughed it off cuz he sees that behavior as him being edgey and charming. Last time I was out with him, he sent back his drink order twice because "Sweetheart, this isn't how I like my old fashioned." On top of that, he's often 45+ minutes late to everything. I don't know how to answer his question except with the truth: your personality and behavior just doesn't mesh well with most people and we just don't enjoy having you around. So WIBTA if I do? Or should I just say that I don't know.