You should find a qualified herp vet in your area and schedule an exam. Once the snake defecates, put that in a ziplock baggie into the fridge so they can do a fecal float for parasites. Most likely, if they were feeding it wild mice, the snake is going to be loaded.

Best thing you can do in the meantime is set up a nice sterilite tub for the snake.

I would feed him one appropriate sized rat weekly. Meaning, if he's feeding on live, he can probably take a weaned rat (around 60-80 grams).

There's no way to really say if this snake is underweight without pictures, but generally when a snake hasnt been fed large meals, you'll want to start smaller and work back up to a good feeding size. (Which in most cases is a weaned rat or a few mice.)

I would offer him rat pups to start and get his metabolism back up to speed, and every week offer something just a little bigger until your back to a weaned rat size.