Definitely get to a pet supply store and get "Nature's Miracle" or some other similar product - it has an enzyme that breaks down the chemical that makes pets think "Aha! Good spot to pee!" Clean every spot with that stuff.

Then be very vigilant - take him out often, don't go back in until he's "done his business." Bundle up if necessary, I do understand it is January out there!

Is he neutered? Are there any new dogs in the house? Any other recent changes in the household he might be reacting to?