[Help] Need advice about elderly dog we rescued

We rescued an elderly pit bull mix, he is 15. We can not get him to go to the bathroom on the concrete outside, around trees, or fire hydrants at all. We live in Midtown Manhattan, and grass is not an area. We took him this past weekend to a friend's cabin and he acted like a normal dog! Only going in the grass, going first thing in morning, right before bed. Then we get back to the city and we can walk him for 1.5 hours in morn, 2 hours at night, middle of day, and no actionn except when he can't hold it any longer, and that is indoors. He suffers from slight dementia, and again is 15 so we are very patient, and accept that he will not always be in control. But after a weekend where he always went in the grass, and was on a schedule, we just can't figure out how to reproduce that where we are in a an apt. So we are seeking advice how to inspire an elderly dog to got to the bathroom within a block or two inside a city. Thank you for any advice. via /r/dogs http://ift.tt/2qC8zmU