The 53-year-old is working with a fitness guru to get him sweating in the gym and help him cut down on smoking, drinking and eating sweet treats, reported Showbiz spy.
"He's doing everything he can to look good. It's no secret that he's a bit vain, and he knows he's got to work hard to stay in shape," a source said.
Cowell is reportedly dating model Louise Griffiths, who is 19-years his junior, and it has been said that he has started the new exercise regime to get fit for her.
"She and Simon have started to get very close. It is early days but there is certainly a chemistry between them," it added.