Chihuahua Peeing in the House?

My dog that is about 4 years old (I have had him about two). Recently he started urinating on my stuff in the house. One day, he even crawled on my bed (he is not allowed on our bed) and urinated on my side of the bed. We also have another Chihuahua who does not pee in the house. ( I don't think.) I am pretty sure that it is mine (the other is the boyfriends).

We have had them together since I got mine, they are best friends. I am not sure what to do. I am so frustrated. I do not know when he goes all over the house. We let them out often and reward them when they come in. Please help.

By Jenny