When you were preparing your client for his deposition, you probably told him not to volunteer anything that the opposing lawyer didn't ask about. Asking questions yourself would violate your own instructions to your client not to volunteer. This is one of the reasons why you rarely see lawyers asking their own clients questions at a deposition.
There are exceptions, of course. If your client's testimony on a key point was inaccurate, you can attempt to repair the damage with your own questions. If your client's testimony in a key area was damaging and needs to be explained, asking questions can allow your client an opportunity to give his explanation. Keep in mind, however, that if you ask questions for either of these reasons, it will send a signal to the opposing lawyer that you didn't prepare your client well enough and think you've been hurt by his testimony. Don' take this step without thinking about it first, especially since your questions probably won't repair the damage.
Another common reason to question your own client is because you need to elicit facts that will be used to defend an upcoming motion. Since testimony at a deposition will carry greater weight than an affidavit when attached to a legal memorandum--psychologically, if not as a matter of legal rule--it often makes sense to establish the facts at your client's deposition. In sexual harassment cases, for example, it is common for the plaintiff's lawyer to ask questions that will be used to help defend a motion for summary judgment.