Jack Osbourne has been literally fighting to lose weight and his proud rock manager mom Sharon says he's winning the battle big time.

Thanks in large part to Thai boxing training, the 19-year-old, who beat a drug addiction in 2004, has shed more than 40 pounds of flab and now looks fab, his outspoken mother said.

"You will not believe what Jack looks like now," Sharon told The National Enquirer in an exclusive interview.

"If you look at before and after photos you will be shocked. He's gone on a rigorous six-month mission to get fit and he has." The 5-ft 9-in celeb who once tipped the scales at 215 pounds has slimmed to 175.

Sharon said Jack has also given up drugs and booze, staying on the straight and narrow since he entered rehab for an Oxycontin addiction last year.

She continued: "As we speak he's 5,000 feet up a mountain in Northern California. And he's on his way to 10,000 feet tomorrow. He's doing an extreme sports show for British TV called Jack Osbourne's Exciting Ways To Die. He's done everything from mountain climbing to bungee jumping to kickboxing."

Sharon said that she and legendary rocker husband Ozzy were in Thailand in late April to see Jack's bout in the ring.

Jack surprised the crowd by knocking out an experienced Thai boxer known as "The Man" just eight seconds into the second round.

He trained for more than three weeks at the elite Fairtex Muaythai Fitness Camp in Bang Plee Yai near Bangkok to learn the traditional Thai martial art Muaythai, which combines both punching and kicking. His trainer Philip Wong, chairman of Fairtex and manager of several Muaythai world champions, said: "I was very impressed with his training. He behaved and he trained so hard for it... He lost 20 pounds when he was with me. His strength is his punches and he has a fighter's heart."

Sharon said: "He's dropped another 20 pounds since we were in Bangkok. Jack just looks incredible now."

To prepare for his rigorous Muaythai training regimen, he underwent a detox program on Koh Phangan to cleanse his body and lose weight. The program included strength and fitness training as well as daily colonics.

Was his mother scared about her son going head-to-head with such an experienced fighter? "At first my parents were concerned," Jack said. "But once they saw that I was prepared — they were very cool about it."