You need to train him like you would a pup. Take him out often, when he pees outside, praise like a lunatic!

Watch him like a hawk in the house, if he "assumes the position" quickly clap or do something to distract him and rush him outside. Clean every spot he has peed in the house with "nature's miracle" or some other similar cleaner that has an enzyme that breaks down the stuff we cannot smell, but makes dogs say "Oh, yeah, this is a good pee spot!"

Has he been to the vet yet, to rule out a Urinary Tract Infection that might make him prone to peeing often?