As long as you are saying no in a firm voice (not loud and angry...boy that's a hard one sometimes) no is making an impact. Everything I have read says that babies that small understand no, but usually don't listen. I think it has to do with testing boundaries. So, just make sure you are firm, and reinforce the no, with removing them from the situation or taking away whatever they have that you don't want them to have. Things like that. Sometimes it's really hard not to laugh when they are being so cute, but try to remember that will only make him think it is a game.
Neither of my kids were/are talking at 13/14 months, but sometimes I could get the correct response from me saying "no" One thing we try to do, is when they do listen, make a big deal about what good listeners they are. I have a 2 1/2 year old and a 13 month old. If you tell him things does he understand? Like pick up that toy? I had been a little concerned with my son that he wasn't talking, and that maybe he had hearing issues, but the pediatrician said that as long as he responded when I said something in a way that let me know he heard me, not to worry. Kids that small are still figuring out the world around them, testing, discovering, learning, they just don't always do it the way we think they are going to.
So good luck, and just be firm. No will mean no and get the right reaction soon.