My stepbrother peed on my bed...how do I clean the mattress?

The same lovely boy who drew a picture on the wall with lipstick came up to me and said this:
“Mads!! I drew you a picsure!”
“Oh, thats nice! May I see it?”
“It’s in your room.”
“Really? Lets go look!”
“I made it with peepee!”
”....what.”

So I go and look, and sure enough, he peed. On my bed.
I threw everything that was on the bed in the washing machine, but I have NO idea what to do with the mattress.
help