Why does he do it?? (my puppy)?

He's marking his territory. He wants to make sure he's the dominant male in the pack. You'll have to break him of this fast. Soon he'll be peeing a river. He's a big puppy. Take the sheets away for a while or bleach them so he won't smell the other dogs on them and see if that helps. Has he been neutered yet? Sometimes that helps with male dominance issues.