Night feed at 1++yo

I trained my 29 mths when he was 5 months old to sleep through the night without milk. initially he will wake up n whine but we don't care. we just feed more last feed and 1st feed. very difficult to train my 2nd son now 9 mths old coz he will wake my eldest with his cry. last feed 9.30pm, then 2 and 6am... super siong.