I’m sorry that I don’t know what you can do to fix it now that your kid’s name is registered like this.
For the benefit of future readers, as far as I know you MUST insist that the child be registered with both Chinese name (characters) and foreign name in the HHR from the get-go and no pinyin version for his “foreign” name (that’s how my son’s name appears in his HHR). In order to accomplish this, you must have the English-version birth certificate from the hospital where your child was born - and that means getting it before mom checks out after the birth.
If you’re successful in changing the name, please post back, and good luck!