My kid was fat at 12 years old but leaned out by:
1. Getting into swimming and being on the swim team.
2. Puberty. Lots of chubby kids lean out when they hit puberty, but at 4'5" and 200lb, its probably going to take more than puberty for your kid.
My son's friend was fatter than he was, and the mother used to worry about his weight and nag him all the time. Then you'd look in her shopping cart and see oreos, chips, ice cream etc. She'd yell at him for eating it but keep it in the house.
So yeah you are on the right track. Get him out of his parents house, get him into some kind of sport that he will like, and get him started on healthy eating and exercise habits.
For sports, I was lucky my son was not self conscious about being a chubby kid on the swim team, but you might have trouble getting this kid involved in team sports if he is self conscious. So, you might consider some individual type activities like hiking, bike riding etc. But team sports really work well because they force you into a routine and push you harder than you would on your own.