I am surprised you were unsuccessful in getting hold of crutches so far. While you can try at the pharmacy at Villaggio, I now beginning to have doubts you will find one there too. As such my suggestion is for you to call medical equipment supplier companies in Doha and check with them. They may be able to arrange one quickly should it not be currently in their stocks. Here is link from where you can get this information.
https://www.google.com/search?source=hp&ei=-IPJW_60I4j9rQG8hYLgCw&q=medi...
Meanwhile, I will personally make some efforts to help you get one. By the way, how old is your son?
Please keep me posted on the progress in your efforts. You can PM this to me.