This is so similar to me as a child, it's weird. I spent almost every weekend at my grandparent's house for most of my childhood. I had a cousin who is almost 4 years younger than me, and we spent an awful lot of time together as kids. We grew up together. And I always wanted her around too. I'd ask for her to stay with me at my grandparent's house... or I'd stay at her house. And I distinctly, and sadly remember being SO mean to her! All she wanted was my attention and to play with me, to do what I was doing, be like me, etc., and I vividly remember brushing her off, giving her attitude, getting smart with her, etc. over the littlest things! Thankfully, she doesn't remember it... we are the best of friends now, more like sisters. I really don't know why I did that to her. I was a really well behaved, well mannered child. Relatives used to take me to nice dinners with them, on vacations with them, etc., because I was so good and easy as a child. I'm certainly not bragging by any means, only pointing out that even a really 'good' child can act like a fool sometimes!! To this day, I don't know why I did that to my cousin. Maybe it made me feel superior in some way? Just wanted to offer some reassurance that maybe this is kind of normal in some way, maybe the age difference contributes? But rest assured, if they're already such good buddies, as they get older, they will most likely only get closer.
I do agree with the other posters, though, that maybe you should tell him no co-sleepovers unless he is kind to and shares with his cousin. I don't think our parents paid close enough attention to us to know anything was going on (LOL), so kudos to you for at least being aware, and nipping this in the bud!!!