He's nine years old, but he suddenly started peeing on the leg of a chair in a certain room, the backroom on a gift bag, and the corner of my rug by my bed. I put down those pee pads for him, where he usually goes, but he just finds a new spot! I don't know what to do, he isn't neutered but he's the only dog here. I have a cat, but we've had both of them since they were little so they basically grew up together.
There is a reason why your dog has started to urinate everywhere, and whether that reason is behavioural or medical, you need to take him to a veterinarian.
There are many possible medical causes, ranging from a urinary tract infection, joint pain, metabolic disease (diabetes or Cushing's), prostatic disease (since he is intact), to something more sinister. Equally well the inappropriate urination may be a behavioural issue, such as marking behaviour or anxiety due to a change to his environment.
Either way your veterinarian will need to rule out a medical cause, and at minimum will likely want to do bloodwork (CBC and chemistry) as well as a urinalysis, and may want to do pursue imaging.