Unfortunatly, this isn't the first time he's done this

I really don't understand why he does this because HE sleeps on my bed w/ us. He's able to jump on and off the bed, so I don't think it's because he's stuck up there. He doesn't do it often, but once in a blue moon he will do it

It's just before he did it during the day so I had time to wash the blankets. Nothings gotten onto my mattress yet (crossing fingers) but my boyfriend did mention buying a new one just incase. I think I'm gonna have to get plastic sheets!
Anyone got any advice on how I can stop him from doing this???