There are many reasons why a boy would have a confusion about his sexual identity. It's very complex and mixed up with lots of emotional, social, and hormonal factors. Sometimes boys have feelings like this, especially around early puberty, and then develop out of it as they grow older. Sometimes a person really does feel that their emotional identity is at odds with their physical gender.
Here is my advice:
First, tell him the physical facts about the differences between boys and girls, and the biological functions of the different sexes. Age 10 is way too old for him to still be in the dark about this, and his ignorance may be contributing to his feelings.
Second, make an appointment to start seeing a psychologist that specializes in youth and adolescent issues. The psychologist can help him deal with these feelings, and help him sort out what is driving the issue. He will need to see one anyway if he continues to feel he is the wrong sex, because they don't do sex reassignment surgery until they are very sure that is the right way to deal with the situation. Some people may present with the complaint that they are the wrong sex, but the real issue is something else that is just manifesting that way.
Finally, it is very rare that sex reassignment is done before puberty, except in physically intersexed individuals and it is diminishing even in those cases because the children are not able to give informed consent. There are other ways to deal with some of these situations than choosing a sex and forcing the body to conform with the conventional definition of one or the other. It is especially rare in children who physically are unambiguously a specific sex. Because there are so many emotional and hormonal influences that can influence self-image, it's better to wait until after puberty completes to avoid misinterpreting difficulties of adolescence. EDIT: I wrote the last paragraph in haste, based on what I knew about the subject about 20 years ago. I don't know how they are handling transsexualism these days. It's conceivable that they may begin sex reassignment before puberty if warranted. It might work better if they start before the body begins to mature, so they won't have to reverse the results of maturing into the gender that is not desired. Talk to the psychologists or other experts to find out what treatments are offered these days.
Try to give your boy the freedom to work through this without undue judgement or coercion. There are many possible outcomes, try to focus on finding the result that will allow your child to develop into the best adult he can be, whether that adult is male, female, or an intermediate sex. It may also help you to contact a Lesbian/Gay/Bi/Transgender (LGBT) organization in your area to get more information on the issues involved, without judgement or undue pressure. In the US there is an organization called PFLAG (Parents and Friends of Lesbians and Gays), there may be a chapter or a similar group in the UK.
Regarding the high school issue, I once saw a fictional story about a family with a strongly transgender boy that they had finally allowed to dress and act as a girl, because he was miserable being forced to maintain the male role. (An episode of the TV program called Judging Amy.) It became a legal issue because the boy's school had a conduct issue with the child refusing to act in accordance with his physical gender, causing disruption and leading the school to suspect child abuse. After an interview with the child about her self image, Judge Amy suggested that she "pretend" to be a boy while at school, and she could act as the girl she felt she was at other times. You might pursue a similar solution.