Depends on whether he bills against the retainer or not.
A retainer, technically, is money you pay your attorney just to take your case. Then you pay hourly for any work on top of that.
However, many attorneys (I'm one of them) will bill against it, meaning that they would charge at their hourly rate up to the amount that you gave as a retainer. Say, for example, your attorney charges $200 an hour. Then after 10 hours, that would mean that your retainer is used up. He would ask you for more money at that point.
If you didn't use 10 hours worth of work, however, then there would be money that was unused and therefore could be refunded to you.
I have given refunds to several clients over time when their cases were resolved without spending down the amount of the retainer. However, most of the time I've used up their retainer on their cases and then some, so they get a bill, not a refund.
I've heard of attorneys who take huge retainers, do little to nothing, the case gets settled by the parties quickly and then the attorneys won't refund anything. I consider that unethical, but if it was truly a retainer only and the attorney was clear that he didn't bill against it, then there's not a lot you can do except refer your friends to other attorneys.
Call your attorney and simply ask if there is a refund coming your way.
Hope this helps answer your question.