When he started the New Zealand certificate in exercise at UCOL in February, Callum weighed 138kg, but is now under 130kg.
"I have got so much out of this. Prior to the course I didn't have any real focus with fitness goals and I had no personal training ambitions," Callum says.
"It has taught me a lot about the human body, anatomy, nutrition and motivating myself and helping others to get motivated."
It has taken Callum a long time to find something that sticks. He tried studying graphic design. He tried music. He worked a variety of different jobs, with mixed results.
"I had a few issues back then with applying myself. I was younger and I couldn't see where qualifications would take me, but that's all in the past and is sort of beyond comprehension now."
The course has been mapped for students to get personal trainer accreditation, which is the highest level you can get within the Register of Exercise Professionals framework.
Despite only beginning his weight loss and training journey at the start of this year, Callum is already passing his knowledge on to others.
As part of the six-month, level-four course, students work with Inspire Health and Fitness Centre on Wilson St where they take on clients.
Callum has retained his client for eight weeks and says training her has been rewarding.
"Initially, it was all so foreign to me because I've never trained somebody or helped them towards their goals. In the beginning I was shy and quiet.
"Now I'm able to push her further and I'm not afraid of it, week-by-week we've built a friendship, she has shown real improvement and is enjoying it."
With four weeks of the course to go, Callum has his eye on local and nearby training facilities.
"I'm a lot more focused now, I know there are limited opportunities in Whanganui, but I don't have that negative attitude of 'oh there's nothing there'.
"I will do my best to find what is available."