
I think you need to find out if this is a peeing (holding a full bladder) or marking problem. As you are aware most male dogs like to mark their territory, and unfortunately many do this indoors too, as they don't know it is unacceptable.
Sadly he may have learnt to mark indoors due to being insecure. A bit like making a hotel room more homely by putting your things out on the dresser.
My freind had this problem with a houstrained puppy when he reached adolescence. What cured him was crating him, when he couldn't be watched. As she worked she was very loath to do it, but we teamed up and I went in and let him out every two hours in the day. After a couple of weeks he had stopped doing it.
This is the one instance where you should not ignore accidents if you catch him in the act or attempting to leg lift, stern disapproval and take him outside and praise if he performs there.
Of course it could be that he just can't hold his wee for too long? Are you finding a big puddle or lots of marked places? It would be worth having the Vet check him out to check there is no urinary infection.
Something that seems to be used in the USA is something they call belly bands. they are either a belt that holds a sanitary pad that is fastened around the dog covering his peeing equipment. or just an elastic bandage holding an absorbent piece of towelling (a face flannel would do). The dog finds it wets itself when lifting his leg, so is less keen to do so, and seems to work for some. Obviously you could not leave a dog like this for any length of time as he could end up with urine scalding.