I have 2. Both are 6 and 7. Little girl (6 - had since she was a puppy) doesn't ever really pee indoors. If she ever (rarely) does it's by the back door and you can sort of tell it's a case of "I was here, where the f**k were you!?!?".
Little Boy (7 - had since he was 18months old). Oh I have cried over this dog. I don't think he was ever fully toilet trained as a puppy. Someone told me that our cleaning products often contain ammonia which to them smells like pee so doesn't remove the smell. This to me makes sense, so I used biological powder like Daz as advised to clean it up. Literally scrubbed EVERY corner. He will pee in doors and mark. Not always, but at the time it was frequent enough for me to resort. Scrubbed everything, spent hours doing it, then rewashing the floors twice to get rid of the residue. Came down the next morning and he had literally gone to town. OH came downstairs to find me sobbing hysterically in the shower and threatening to give him away.
Please do not jump on horses at that last comment. It's just so that people know some background. I was so so SO upset. It was the exhaustion talking. But never in a million years would I give any of my dogs to anyone. They are literally my whole world and when I thought I had nothing, they were there.
Over the last year he's got better, I have literally let the little dude out every time I've passed the back door. We recently moved and I noticed him slipping up more, so I got a dog trainer in. Apparently people tend to think of dogs as humans (very guilty) and neglect the 'dog' needs of a...dog. Mine are bored, they don't really play for long other than with each other and are lazy by nature so love a cuddle. Turns out I've probably made them depressed. I've been doing 20mins per dog of training per night with what he's taught me, and her constant licking of sofa/clothes/the air/anything around her and him peeing are decreasing.
For those that say you can't teach an old dog new tricks I honestly beg to differ. I have 2 stubborn old pugs. Who now sit and lay down when commanded (with treats, we're still in training). He also told me to almost go back to basics with toilet training for little boy. So now, I open the door, I go out with him and say "go for a wee", I don't watch him as such but I make sure I am aware (apparently I am not to follow or force him too, just say Go for a wee a few times) when he goes, and I literally do the whole "GOOD BOY" thing. It's working-ish, but it'll take a long time to sink in. We're getting there and I won't give up.
Another very useful tip he gave, was never let them see you clear it up. Just calmly put them outside and clean up when they aren't there. Don't make a big deal about it, apparently unless you catch them doing it, they don't understand that it's that what you're telling them off for. They have no concept of how much time has passed and they don't associate the telling off with peeing. If they see you cleaning apparently they'll think it's normal for them to do it and that you aren't bothered (not sure on that but i'll try anything).
Also, food, take a look at Millies Wolfheart, available online. My two have all sorts of skin issues and this stuff is amazing, and it's very nautral. I do notice accidents increase if I'm later than usual feeding them. Not much you can do about that if your day over runs.
Maybe a dog trainer will help? Do stick with it, he's so young and he'll still be so quick to learn. You can normally get one come to the house.
Good luck, don't give up!