First off, a 90 gram weight drop after a 4 month fast really isn't too bad. As long as he's got fresh water at all times and can drink, he should be fine until he's ready to eat again.
Now,
Feeding live is pretty simple; the snake does most of the work
Just monitor the feeding after you drop the prey in the enclosure. Don't worry about feeding live, Ball Pythons are born to wreck rats. Just keep an eye on it just in case your snake still doesn't want to eat. If your BP hasn't struck and killed the rat after about 20 minutes, I'd remove the rat and try again in a week. And after a hunger strike, sometimes it's a good idea to start out with a rodent that is a little smaller than the snake is used to.