stopping leg lifting and urinating

Question
My bug(boson terrier/pug mix) is just 7 months old and he has started to lift his leg and urinate on the walls and toys in our home.  He is potty trained--so not sure why he is doing this? Ive had other males before, and never had this problem. Is there anything I can do to stop him--I never catch him either--I find the mess and dont want to scold him after the fact. Help!! He was just neutered in hopes that this will curb him!! Thanks

Answer
He is marking-he is just reaching the point when he gets adult urges and is displaying a natural territorial instinct.  Keep him in a crate when he cannot be supervised, and clean any areas with an enzymatic cleaner.  When he is unable to practice this behavior because he is either crated or monitored, it will decrease.