Good thing they caught him and got him to the vet; he was already suffering from the same health problems that humans in their late youth and middle age can get from too much junk food..
Also he was teaching the rest of the monkeys bad habits (in spite of presumably not being too fast moving he was difficult to catch as the other monkeys who he had been sharing food with tried to attack the vets and rangers!)
https://www.theguardian.com/world/2017/may/19/obese-monkey-thailand-uncle-fat-diet