NEED HELP, ONE CAT WON'T USE THE BOX - WHY???

I have 2 cats, twins a little over a year old. Simon uses the litter box, and Soloman refuses! It drives me crazy. He is pretty good at meowing at the front door when he needs to go out, so he knows he isn't supposed to go inside, unless its in the box, but if I don't wake up earlier than 4:30am to let him out, or if I don't hear his meows - he finds a towel, rug, or any type of material on the floor that he can find and pees! UGH the smell is horrible and it upsets me because I pride myself on the cleanliness of my home. I can't stand walking into a friend's home that smells like cat urine. It smells like they don't give a care! I do everything I can to get rid of the smell so he won't do it again. But he refuses to use the box. He won't go in the same place every time either! I have one litter box, fairly large, with Cats Pride multiple cats litter. Any suggestions would be most helpful, it is driving me insane!!