Toddler having restless sleeps.

My 16 months old recently didn't sleep so well at night. He will literally wakes up every 2-3 hours every night crying. The only way to pacify him is to give him milk 120ml and he will goes back to sleep. I fed him dinner at 6-7pm and will go to sleep around 9pm. He will have a good 4-5 hours of sleep and will wakes up 2-3 hours after that. Any mummies having same experince or any tips how can I make him have a longer sleep at night. TIA!