The then 15-year-old had been back and forward to his GP with a raft of symptoms put down to gastric problems.
But after treatment for indigestion Caillan was no better - and was becoming increasingly stressed by his health issues.
He said: "It's weird but I wasn't that upset when I found out I had leukaemia - it was just good to know what was wrong with me.
"I knew there must be something wrong, but my GP wasn't picking anything up although I had bruises all over my chest and legs and pain in my back and chest.
"My mum and my sister were so upset, but I was kind of glad to know what was wrong with me."
Caillan, now 18, had always been fit and healthy but suddenly started to develop a rash on his body that looked like bruising.
He had severe lower back and chest pain that would leave him breathless.
His doctor diagnosed indigestion and heartburn.
But one night he became breathless, collapsed and was rushed to A&E where doctors did a blood test and discovered acute lymphoblastic leukaemia.
Despite the shock of the diagnosis, Caillin was relieved to discover what was wrong with him.
The school pupil began a course of chemotherapy almost immediately, which he found extremely tough.
As well as the physical side effects of his treatment, Caillan also had to deal with being on the children's ward in Yorkhill Hospital.
He said: "The chemo was horrible at first. I had sickness, extreme weight loss, hair loss, tiredness, severe mouth and throat ulcers.
"I also had loss of appetite, fainting spells, a mini-stroke and muscle wastage, which caused difficulty walking.
"At was first I was treated in a cubicle in a children's ward, which made me feel isolated, and it was also upsetting hearing and seeing smaller children going through treatment."
FORTUNATELY, Yorkhill Hospital has a Teenage Cancer Trust unit where Caillan was moved after his first three weeks of treatment.
He found being with other teenagers a huge help in being away from his family and having to cope with his chemotherapy.
The six-bed dedicated unit has a kitchen and relaxation area where Caillan's mum, Lynn, and 10-year-old sister Emma would come to visit.
Caillan, from Lennoxtown, added: "I was moved to the Teenage Cancer Trust unit, which was a lot quieter and had staff that were used to treating teenagers, unlike the staff in the children's ward.
"It was also good to be in an open ward which allowed me to communicate with other teenagers around my age which made me feel more relaxed.
"We had days out with the youth support coordinator which brought all the teenagers together in an environment which wasn't in a hospital.
"That made me feel comfortable and to be able to socialise with other teenagers who had been through similar situations.
"This helped me a lot as it gave me a confidence boost to see other teenagers who were going through similar things and doing really well."
After three years of chemotherapy, Caillan is now on maintenance doses and should be finished his treatment in January.
Despite missing school to attend hospital he has kept up with his studies - and is studying for an HNC at college and plans to carry on to an HND.
He added: "Teenage Cancer Trust especially helped me with going back to school.
"Missing a lot of school dents your confidence, but the Trust helped with that and now I'm at college with plans for the future.
"With my treatment finishing in January it's a really exciting time."
Caillan is now backing Teenage Cancer Trust's bid to raise £500,000 for a new unit for young people with cancer in the West of Scotland.
It will be based at the new children's hospital in the Southern General Hospital campus.
For information see www.teenagecancertrust.org or text GLA15 to 70500 to donate £5.