meh flip the mattress and move on with your life. It'll probably wash off the mattress cover with enough washing, if not worst scenario they bin it. It's not something you can help, things happen. If he kicks up a fuss or gets grossed out then he needs to grow up a little. Hell I've had this happen with a friend let alone a significant other, It's a total non issue. We made a joke of it in the end, though in all fairness she was shocked and really embarrassed at first as I imagine you are.