A healthy ball python can go a year without food without issue and you do not assist or force feed an animal that knows are to eat, adding more stress to an animal that is off food is not gonna help. Many vet with limited experience will jump the gun not understanding what can actually be done to help an animal resume feeding.
What needs to be look at closely his your husbandry
It is not unusual for a sexually mature 3 years old snake to go off feed now getting more details will help.
How big is he (weight)
What size and type prey were offered prior to going of feed.
How big is your enclosure?
What are your exact temps and how do you measure them?
What type of bedding do you use.