J.V.
I am a worrywort anyway, but I would be concerned. Is it possible someone (an adult) is making him uncomfortable at these sleepovers? Do your cousins have friends over that he is not familier with? You are not there to see what is going on. Ask him if anyone is being inappropriate or making him uncomfortable. He is old enough that you can talk to him. I find the best time to talk with my children is when they are going to bed, the lights are out, I am sitting next to them and they are relaxed. They will seem to tell you more then, bad and good. Good luck, 4/12 is such an awesome age!