D.C.
My grandson did the same thing,, only he peed so heavy that it came through the other side,,of the futon,,what I did was,, I took the mattress out side,, and put it up on two chairs,, and I got a bucket ( 5 gallon bucket )and filled it with water and I dumped it on the futon where the pee was,, I did that 4 times,, then I refilled the bucket and I added 4 cups vinegar to the 5 gallon bucket and dumped that on the futon mattress,, and let it dry,, for a couple days,, went out and saw that is was all dry,, smelled it,, and you know what,,It smelled so clean,, not a whiff of pee was there,, all clean,, try that,, let me know what happened? only good could come out of this,, k? good luck,, D.
P.S you can also add fabric softener to the rinse water,, if you want to,,