My impression of the OP is that this baby is still being assist fed. If I'm wrong and he's now eating on his own then he'll make up for lost time soon enough.
Otherwise, while ball pythons can be picky that's normally true for older animals; unless there is something wrong with their environment or they are stressed babies typically eat. So, before going to the vet, a few questions:
- What kind of setup is he in (tank, tub, PVC enclosure)? Does it have hides for your baby to get under to feel secure?
- What are the hot side temperature and cool side temperature? What kind of thermometer are you using to measure them?
- What kind of thermostats are you using to regulate the heat sources?
- What is the humidity in the enclosure and what type of hygrometer are you using to measure it?