Consistancy is the key to all training. No matter what you are teaching, do it the same each time, using the same words or actions, and reward good behavior always, as previously stated. It helps to use the same door for going outside to potty all the time. I currently share my home with two dogs, a dobe (my fourth) and a dobe mix, both were shelter rescues. The mix was in a no kill shelter for five long years with no socialization. He had a lot of emotional problems and had never been in a house before. He was completely housetrained in one month, just by using the same phrase and the same door consistantly. The phrase I use is "potty outside". When I would take him out, I would always ask if he had to potty outside, then when he did go outside, I would praise him and make over him big time, telling him he "pottied outside and he was a good boy". If he had an accident in the house, I didn't yell or spank, I just told him no, and said potty outside and took him outside immediately. Then while I cleaned the mess, I wouldn't talk to him at all. But when I was through cleaning, it was like it never happened. As you work more and more with your dog, you will see the progress and have more patience for it, especially when you see how your dog responds. I may be a little biased, but I've been working with dogs for over 23 years and I'm convinced Dobes are the smartest.
I also agree you should have him neutered if he isn't yet.