My 1 1/2 yr old male chihuahua (not desexed) has recently started peeing on my pillow, and the mattress?

Is it possible that while watching the other it urinated there? If so that could be it. Dogs will smell it and assume it is ok to go there. You will have to retrain him not to do it as well as completely remove the odor. There are sprays that are supposed to remove any trace of odor. And like the others said if it is just a little he is just marking. Good luck.