At 4 months old, you can probably feed him an adult mouse or a rat pup. Then you don't have to bother with the double feeding.

If he is feeding strong on live, chances are he will convert to f/t without any issues. You might have to hold out for a few weeks til he is really hungry, but I've never had any problems switching ball pythons of any age from live to f/t and sometimes back and forth. So if you wanted, you could offer him the occasional live meal.

I would give him live for his next couple meals just to make sure he is eating well with you, and then offer some F/T and see how it goes.