A couple of things come to mind. When you say catch him in the act...are you catching him the instant he starts and not letting him finish of is it really after the fact?. I catch him in the act and reprimand him and take him outside immediately but he just does it again.
Are you rewarding and praising for going in the right potty spot...really teaching him that that's the correct place to relive himself?
As you stated, he's sort of house trained....he shouldn't have access to the rest of the house until the training is done.