How to get my dog to go to the bathroom in his own yard?

So I've adopted a 4 year old German Shepherd and he is housebroken. Will not go to the bathroom indoors at all. But problem is he also will not go in my back patio. I live in a townhouse that doesn't have a gred backyard so I have to walk him over to the park twice a day for him to go. He will hold it to near breaking point, whining and crying, but refuses to go on the concrete, once I let him outside he will just sit at the door and watch me with a "what" look on his face.. I'm proud of myself for sticking with the 2 walks a day thus far, as they are good for him, but it would be more convenient and realistic that I be able to have that option of letting him outside real quick to maybe finish errands within the home and let him do his business without me having to drop everything and walk him. Does anyone have any ideas on how I could get him to used to accepting the patio as a bathroom?