Josh Scott-Hill, 11, has been growing his hair for 18 months after seeing a friend lose hers during chemotherapy.
He said he had been "teased and mistaken for a girl".
Josh will give his hair to Little Princess Trust which makes wigs for children, and has also raised more than Â£1,000 for Maggie's Cancer in Swansea.
"During the time I have been growing my hair I have been teased and mistaken for a girl so many times I've lost count," he said.
"There was only one time I got really upset, but luckily my friends are very supportive as are my family."
His mother Sam Scott said she was very proud of her son, who will have his hair cut on 15 July.
"He is looking forward to it - but I love it long now so it will be a sad day for me but I'm sure he could grow it back," she said.
"I think he would do it again, I'm so proud of him."
Ms Scott added Josh was inspired after seeing a friend of hers after chemotherapy.
"My friend has had cancer and she lost her hair and you know kids, she was wearing a cap but they were asking 'where have your eyebrows gone?'
"When we walked away Josh said 'mammy I want to grow my hair for people less fortunate' and I thought 'oh right, OK then' but he has stuck with it.
"He gets mistaken for a girl all the time, he does have quite a pretty face, and he has to tell people 'yes, I am a boy', a lot of people are quite shocked he wanted to do this.
"But he has got used to all that now, if anything it just made him more determined than ever."
A boy from Llanelli has grown his hair to about 25cm (10in) to make wigs for children who have lost theirs due to cancer treatment and other illnesses.