why can it not be "can"? He can, no Problem. It was also can he have chocolate?
'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'
No reason at all.
It is usual to include 'yes', but not essential, and then he/she, and then may/can. The database has now been amended to cover all those options, although it may take a little while to go fully live.