The 43-year-old funnyman is said to have hired diet coaches and has given himself a goal of losing at least 18lb, reported Sun Online.
"A few years ago Jack was one of Hollywood's most wanted stars but things have gone off the boil a little. He has been told there are some parts he's just too large for and has decided to try and lose some weight.
"He loves junk food and is going to have to kick that habit and start working out too. But he will make sure he doesn't lose his sense of humour too," a source said.