Abnormal behavior?

Has anyone seen some abnormal behavior in someone when they drink? I know what drunk people are like, this is different. Every time my cousin (21M) drinks recently, something weird ends up happening. He wasn’t always like this. He shows weird behavior when sober too, but it’s amplified 10x when alcohol is involved.

For example, often times he will end up face down, ass up bent over a bed or chair of some kind. Another time recently, our parents were in town visiting our university for a home football game. When we got into the stadium he just “wandered off”. I was tailgating with him all day and he didn’t even drink that much. But his dad had to get the police to track him down. I still don’t fully understand the situation but I don’t want to ask him about it, I assume he was embarrassed. He’s been secluding himself more and more and barely hanging out with people these days. Every time he drinks these days something odd ends up happening with him. He’s starting to be oblivious to simple social cues. And he’s become hard to talk to. He almost comes off as a sort of Joe Biden like not all there mentally. He should know how to handle drinking at this point. We’ve both had plenty of experience. He seems to be experiencing some sort of gradual cognitive decline.

Anyone know what this may be? I’m assuming some sort of depressive thing. But this has been a gradual decline for over 2 years at this point. I feel like I can’t talk to him and have a real conversation anymore. I can provide more detail if necessary