I run a small reptile rescue out of my home and the oddest thing is that I see more morbidly obese Savannahs than I do sick,emaciated ones,the reason being is that monitors will eat just about anything you put in front of them,it's almost like a wierd case of neglect,the previous owners neglect them by not putting in the effort to obtain proper food items and instead reach for easy to obtain things like dog food,which is recommended by some,adamantly opposed by the experts in the field such as Pete Strimple,John Coborn,etc...
This kind of feeding attributes to most cases of obesity and cases of death where fatty acids build up in the liver and cause failure,one of the worst cases was a Savannah monitor that came through my rescue,it's surrendering owners were feeding him Vienna Sausages (those little canned sausages,packed full of fats and sodium),canned dog food,wet kibble dog food and raw bacon!
Of course we tried to put him on a proper diet but he eventually died anyways though there have been a few others than pulled through alright and went onto become healthier active monitors.
The first thing i would suggest is to raise the cage temperature and limit feedings to crickets,waxworms and other invertebrates,pinky and adult rodents should comprise little of his diet for now,most Savannahs in the wild eat mostly invertebrates really,especially as juveniles,not that rodents,chicks and other such larger prey items aren't valuable in the diet,they should just be less liberally fed than commonly practiced,in my opinion and from what I've researched.
Raising the cage temperature should stimulate him a bit and removing him from his cage for walks and exploring sessions may have some benefit.
It will be a long slow process for him to lose weight,they put it on fast but lose it slowly,they've evolved to hold onto those fat reserves to see them through times when prey is scarce or through illnesses so getting it off will take some time and alot of reduced feedings and exercise!
Good luck to you and your pudgy Savannah Monitor!
StormyHall