My dog will not stop peeing indoors and in my bed?

i have an albino blue pit mixed with american bulldog around 15 months old. i walk him for one mile three times a day, limit his water intake, and take him out regularly in between walks. once i turn around he pees in my bed or on the floor, ive tried scolding him, smacking his nose lightly, sternly say bad dog, taking him out even more often(20+ times a day). i'm running out of ideas, the vet says its not a bladder infection. i pay attention to him constantly so i know its not an attention grabber. i love my dog but i cant spend every waking moment picking up pee and washing my sheets. please if anyone can help me id love to hear some other ideas, i'm confused and am actually thinking of giving him away/ putting him down. (i really dont want too!!!) thank you