keep doing what you're doing. it took us almost a month to potty train out dog when we adopted him. you just need to be patient with him and if necessary, put up a barrier to keep him only on the grass when you take him outside to go potty. eventually, after having no other options except to use the grass, he'll begin to prefer it over the concrete. good luck and have patience! :o)
If he is a good dog and fairly smart, I would give him time. Hes probably confused right now. Just keep taking him to the same spot your other dog has peed on, when we had two dogs the one always had to pee on the exact spot the other one did, I think it was a dominance issue.
This does take time, it does not happen overnight.
Training a dog takes time and patience and You must stick to the routine.