Hi mummy, this is according to my own experience. My Deedpoll fees was around $150 4-5 years back. And depending whether your son has already taken an ic, that would be another $70-$80. That is all the charges you need to pay as you cannot change the name on the birthcert. I’m not sure about your situation but for kids below 21, do note that both parents have to be present to sign the Deedpoll. :)
Fion Ho