Praise be to Allaah.
Fasting is obligatory for a person if he fulfils five conditions:
1- He is a Muslim
2- He is accountable (mukallaf)
3- He is able to fast
4- He is settled (not travelling)
5- There are no impediments to fasting
If these five conditions are met, then it is obligatory for a person to fast.
Kaafirs are excluded from the first condition. The kaafir is not obliged to fast and his fast is not valid. If he becomes Muslim he is not obliged to make up fasts from before.
The evidence for that is the verse in which Allaah says (interpretation of the meaning):
And nothing prevents their contributions from being accepted from them except that they disbelieved in Allaah and in His Messenger (Muhammad) and that they came not to As-Salaah (the prayer) except in a lazy state, and that they offer not contributions but unwillingly