Marking his territory constantly even though his fixed

Dingo is 8 yrs old and has been fixed & pad potty trained since he was probably 2 to 2 1/2 months old. However, he feels the need to mark his territory all over my house or any where we go, which limits me where I can take him. I will shampoo the areas and use special products to get rid of the smell. It works for awhile and then eventually he goes back to those spots and marks them again. He has ruined all of the bottoms of my doors. Thank goodness I have a shampoo or my car pet’s would be ruined as well. How can I stop of this terrible habit?