Do you have a crate? I would crate train him. Here's an article that explains why this is such a great idea.
I used to think that crate training was cruel (not any more!), but that was before I had my own dog. I was able to house-train Tucker in a matter of weeks. But, more importantly, the crate allows me not to have to keep an eye on him *all* the time, which is tiring. He's 95 pounds trouble.House training in concept is not that difficult. In practice, it's quite the opposite sometimes. What you have to do is not give him any freedom in the house when he's unsupervised. That means you don't turn your back on him for a second. Keep him on a regular feeding schedule and when he needs to go (you'll learn his schedule eventually) take him out and praise/reward him when he goes outside. Take him out as soon as he wakes up in the morning and right after meals. If he doesn't go within 3-4 minutes, put him back in his crate for 20-30 minutes and try again.