The 20-year-old British singer is losing weight because of stress and his busy schedule, the Us magazine reported.
"He's lost a lot of weight. He started to eat healthy a while ago and working out to get in shape as he had never trained before. He wanted some definition," a source said.
"He's always been the slimmest of the band members, naturally but I think he's running around a lot, getting stressed, and not putting health first," the source added.
The group is expected to perform more than 35 shows in the next three months in the US. They're also busy shooting their new movie 'This Is Us' due later this summer.
