I have a 8 month old Japanese Spitz/American Eskimo puppy and we live in an apartment since he was 11 weeks old. He goes out for regular walks and has plenty of space to run, despite having no garden. I have a Cat also, which is male and they are simply in love with each other. He has his puppy pads that are always in the same place so if he needed to go toilet he would, without fail. Only recently has he been starting to wee up things, like a rubbish bag, anything on the floor, yesterday on my bed, and now after coming in from a walk (and I see him pee before coming in) he just pee'd on the duvet cover and pillows! He hasn't been neutured. Why is he doing this?

I am sure others will also give advice but let me start with my thoughts about the pee pad. A pee pad tells a dog its OK to pee in the house, not off to a good start. If he does pee while coming in from a walk it sounds like he is marking his space. If a dog does pee inside the home and you can't correct him in the first second then do not bother. I think Petco is offering free dog training classes this month for house training, maybe worth exploring. Since this is a recent change I would talk with your vet about possible tests for urinary infections.

He is showing his dominance to you and to the cat. I suggest having him neutered.

Marking can certainly be a dominance issue, and neutering is an excellent idea. I totally agree with ballengerb, puppy pads only lead to further confusion. It's allowing him to pee in your house on something soft. Why not everywhere else? Dogs should use the bathroom OUTSIDE. It's important to allow plenty of time out doors for them to completely empty their bowel and bladder. With small dogs and puppies even one piece of kibble or a small drink of water can put pressure on the bowel and bladder, creating the urge for them to need to use the bathroom.

This really does sound like a behavioral problem, and while neutering will greatly help, you need to go back to step one with house breaking. Basically being consistent and on a routine. Have you considered crating while you were away or at night? Crates are an excellent housebreaking tool.