How should I tell my nephew his parents died?

When my nephew was 6 years old, he came to live with me and my husband because his parents died from a car accident. My husband and I decided not to tell him. It has been 3 years since, and every time he asks us about his parents we deflect to spare him of the grief. We figured he was too young to fully understand and hoped that the more time that had passed his memory of them would fade and the less painful it would be for him. He is 9 years old now and we think it is time to tell him, but we have been reluctant to do so in case we should wait a bit longer.

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It has been especially difficult to tell him because I lost my father to a stroke when I was a teenager and I know how painful it is. I thought that I should rather wait to tell him, because knowing the grief I would have wanted to have been spared of the pain for some time as well. I imagine that having been apart from them for a few years would perhaps lessen the intensity of the emotional response. I spoke to a psychologist shortly after his parents died to get a professional opinion, and she told us to wait until he is more mature to process the information and prepare him if possible.

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He has been content here with his cousins, uncle, grandparents, and I and I can't bear to destroy his happiness. I'm worried we've made the wrong decision by not telling him and will regret it. If it was wrong, it was an honest mistake, but the damage is still there.