My account got hijacked and it was used to buy gems in a game called castle clash and he spent alot of money on it and it is urgent I get my money back

Are you able to change the account's password e.g. via email or by answering the account's security questions on http://iforgot.apple.com ? If not then try contacting Support in your country : Contact Apple for help with Apple ID account security

If your country isn't on that page then try this form to contact Support : https://www.apple.com/emea/support/itunes/contact.html

For the purchases on the account, try http://www.apple.com/support/itunes/contact/ or the above form