Dog potty problem behavorial?

[EmmaElizabeth]

We adopted a German Shorthair Pointer about 2 months ago. He is 2 1/2 years old fixed. He has adjusted pretty well, but he marks about twice a week. This is out of control, and I dont know what to do. We let him outside very often, but he will do it even when we are home but very sneeky like. Our other shorthair has never done this, she is perfect in the house(4 years old). The vet said it is behavorial? What can we do? We love him and the kids are attached, but I cant let a dog distroy my house. All help appreciated. Thanks. P.S. the vet ruled out any medical problems.