'He may', or 'he can' in the sense of being allowed to, should be fine for the phrase as it stands. The actual meaning depends on the context in which the phrase has come up.
- Gaiff Siân ddarn o dy gacen? Caiff, dim problem - 'Yes, she can, no problem'
- Gaiff Dewi ddarn o dy gacen? Caiff, wrth gwrs - 'Of course he can'