If your son lies only occasionally, you might talk to him about the difference between lying and telling the truth, discuss the importance of honesty, and help him find alternatives to lying. If he is involved in the problem-solving (e.g. "what else might you have done?"), he might be more willing to try out other behaviors than if you simply tell him what to do.
On the other hand, if he is lying repeatedly, it may be a more serious problem. If this is the case, the first thing to do is find out why he is lying. Sometimes children lie because they fear consequences or because they hope to avoid chores or responsibilities. This may be even more difficult for a gifted child, if he feels that he will be punished if he does something less than perfectly. So the first thing you might want to do is evaluate whether, in reality or in your son's opinion, the expectations at home or in school are too high for him.
Another reason children lie is to get attention. In a society where time is at a premium, it is easy for "time together" to fall through the cracks. If you realize that you have inadvertently been neglecting him, perhaps you could schedule a special time every day to make sure his need to be with you is being adequately met.
Children also lie to get a desired reaction out of other people. It might be useful to try to remember how you have previously reacted in situations in which you learned that your child was lying. The next time the situation presents itself, see if you can change your reaction, possibly by removing yourself until you have had a chance to think about alternative responses. For example, instead of asking him to tell you the truth, you might confront him with the facts that lead you to believe that he is lying.
The most important thing you can do is stay calm and let your son know how much you appreciate it when he does tell the truth. Be as specific as possible (e.g. "I know how hard it was for you to tell me that you broke the window.") Be aware, too, that being caught in a lie can be extremely embarrassing, so take care to reassure your son that you still love him even if he has done something you would rather he had not done.
Your comment about your son still being attached to his comfort doll leads me to believe that you are concerned about his general psychological well-being. If that is the case, you might want read about normal child development (for example, you might read more of the information on FamilyEducation.com or visit the website of the New York University Child Study Center, at www.aboutourkids.org). You might also contact a school counselor or other child mental-health professional in your community for an evaluation -- perhaps someone your pediatrician knows and respects. Sometimes getting a professional opinion will relieve your concerns or at least provide you with some additional directions. Hope this helps.