Since joining a Club Physical gym in September last year and working with a personal trainer, Alder has dropped from 140kg to 80kg - the equivalent of 120 packs of butter.
The Te Atatu student said his personal trainer, Merv Birch, managed to encourage him to lose the weight.
"I've tried before but I didn't do as good," Alder said.
He started with cardiovascular work until his fitness improved enough for him to join boxing classes.
"Ahmed looks a lot different and he acts a lot different," gym manager May Cobb said. "These days he is always happy and smiling."
Alder said he had a lot less stress and the insecurities attached to his weight had gone too. "To be a fit person now is amazing."