Talking to your child about his birth parents may seem like an incredibly difficult task to overcome, but it really shouldn't be that hard. Hopefully your child has always known about the adoption and didn't uncover a family secret. It is completely normal for an adopted child to ask questions about who and where they came from. It's natural to want to know those things. When the time ends comes to have that talk it should be done in passing not as a sit-down event.
Adoptions all have varying amounts of skeletons. Your child could be the product of an unmarried couple or even a rape. The way you answer his questions will affect the way he handles the information. Starting from when the subject is first broached, you should give as honest of an answer as the age of the child will allow. As the child grows up and reaches different levels of maturity and understanding, you may give additional details. As for a sexual assault, the choice will be yours whether or not to disclose all of the facts.
If life has shown me anything though, in all my years it's that the truth always comes out eventually. As parents we want to protect our children from everything. This is their story though. We should teach them that despite their past or start in life, they choose what their life becomes.
However you choose to approach the conversation, it's best done over time. In an emotional situation such as adoption our minds need time to process information before we can start to see things positively. If you give your child news that is upsetting you must allow some time and space for him to deal with it in his own way. Remind him that the past is just the past and now he makes the future.