"It was scary for me putting on weight. Much scarier than losing it because I never used to weigh myself then. I just liked the way I looked," he says.
"It may have started when I asked my parents if they thought I was big. They didn't say definitely, 'no'. They just said it didn't really matter what you looked like. So I wondered."
For the past 22 weeks, following treatment at Great Ormond Street Hospital for Sick Children, Tim has lived at the Rhodes Farm Clinic in North London. The small, specialist clinic is currently treating five boys with eating disorders, the highest percentage of male patients they have ever known.
Tim sees his journey back to health as a change of personality. The boy who idolised Linford Christie and hoped slimming would win him acceptability at school seems like another person.
"I was not bullied every day at school, but I was bullied. It was mainly comments about my personality." As the pounds began to disappear no one stepped in to ask what was happening.
"They didn't notice or didn't see it as a problem. Quite a few people said 'you have lost weight haven't you?' I thought they were saying it was a good thing."
"I got a feeling of emptiness and then, after I had eaten something even really small, I would feel full and awful."
Interview by Vanessa ThorpeReuse content