I have the same case. Let him study K1 first. Next year, you may try to apply those really through train school. Depend on the result and the situation your kid have during his K1 studying, then you can decide again
I still prefer to repeat K1 for kid borned in Dec.The abilities different for a child if they are more than 10 years old may be minimal, but for a kids less than 5 years old, the differences may be obvious. To compete in getting in primary 1, being an elder kids could have an edge. Just my personal opinion