Please share your set-up; tank size, substrate, hide sizes, temperature/humidity levels (and the way you measure them.) It may be a husbandry issue causing him to go on a hunger strike. Pictures help a lot too, so we can see things that we may not think to specifically ask about.

How much does he weigh?
What was the breeder feeding him?
How many times has he eaten before you took him home?
Are you feeding him mouse fuzzies or rat fuzzies?
How often are you offering him food?
Has he eaten since the 3 week strike?

Mouse fuzzies are pretty small, and some ball pythons won't bother with prey that they feel isn't appropriately sized. Though some people can get away with feeding multiple smaller prey items, some BPs will take one food item and refuse the rest, despite having a smaller meal than usual. Even worse, offering multiple days in a row might even stress them out and cause them to refuse food even longer. Hatchlings are also partially attracted to (intrigued by?) the movement of the mouse hoppers, which fuzzies won't replicate.

As far as feeding F/T, it may be an issue with type of prey (he may not want to eat a F/T rat if he's only ever had live mice), maybe the prey isn't warm enough to attract his attention, maybe he's too stressed out by so many different types of food and/or ways of offering it. If he's small from refusing (if he's a month old and refused three weeks, that's most of his life), it may be better to just stick with the prey item that works (e.g. mouse hoppers) until he has a little more weight to spare. Then maybe you could try a more gentle transition, such as live mouse hopper > live small adult mouse > live rat pup > F/T rat pup. Or if rats are easier for you to find, live mouse hopper > live rat wean > F/T rat wean > F/T rat pup. Hopefully others can weigh in with some more advice for you!