[matese]

If you have not already neutered him , do it ASAP before it becomes an indoor habit. If you neuter at a young age, this may help with indoor marking.
I rescued my guy when he was 2 1/2 years old. The prev. owner turned in all his medical papers, my fellow was neutered at 8 mos old. He squats when he pees but will hike his leg outdoors sometimes. He is 100 % outdoor trained, no pee pads used.
Peeing on top of your other dogs pee is ok, he is re-claiming his territory.
I had two females now both passed, the younger one would do that with the older gal.
Check with your vet on what age can he be neutered.
Neutering also eliminates chance of getting testicular cancer, it may add years to his life, also reduces testosterone issues like dominance and aggression.