My son was born in late Dec and had the same problem as you.
I think it depends on the kid. If his development is relatively slow, then probablylet him wait a year. Otherwise, let him try and see if he needs to repeat. I would save the 'repeat quota' for later use just in case.
Many friends told me good schools don't take 細囝. But after the interviews this year, my son got quite good results even from the famous schools. We feel lucky.