Continuously marking and driving me nuts!

Hello….my 4 year old mastiff is amazing in every way apart from his continuous marking. His not been done and feel it’s too late now as it’s learned behaviour. Out on walks he stops every 5 mins to wee, so frustrating but can live with that but he also does it indoors! Never seem to be able to catch him at it. Will especially do it if I bring something new in to the house or move a piece of furniture…any suggestions on how to stop this would be welcome. Have had him checked at vets for wee sample but all clear so no infection, feel like it’s definitely behavioural problem