Well, in the meantime, have you been using a crate all this time? What you do is watch the pup LITERALLY every second. You do not leave that pup unattended at all. Every time he makes a pee pee or poop in the house, that in itself is his reward and will reinforce this.
You need to take him out often, after naps, after meals, after playtime and lots of times inbetween. At six months, he probably should have a little better control, but they all mature differently.
When you take him out, go to one special, designated spot and walk him around a bit. Use a command. When he's in the midst of going, say, "go pee pee" or whatever. Then when he just finishes, immediately say, "gooooood pee pee." Make a big fuss over him....how wonderful he is and give a treat.
If he does go in the house, clean it up when he's not looking and use an enzamatic, pet odor remover to clean. Don't scold him. If you catch him in the act, you can tell him, "no or eh-eh-eh" and whisk him outside quickly. Show him his special spot.
If you're not able to watch him, even if you're just going to go in the bathroom or something for a few minutes, put him in his crate. He won't tend to go in there. You can even tie him onto your belt loops when you're busy, (but want him to have a little freedom), so you can watch him better.
You have to be absolutely consistant. Every mistake (you) make and he goes inside sets him back. This may be why it's taking a while. However, six months isn't out of the realms of norm. My Dobe was about 9 or 10 months before he was reliable. So, hang in there. He'll get it.