6 MO baby waking in middle of night asking for milk

I am puzzled that why my 6 MO baby have been waking up frequently in middle of night asking for milk when he could sleep through the night when he’s 2 month old. I notice ever since I stopped giving him breast milk at his 5 MO, he has been waking up every 4 hourly for milk. I tried to increase his milk intake to 180ml, but he will always stop at 150ml and refused to drink more. What could be the cause? And how do I counter it? This is so tiring and we had managed to make him sleep through the night when he’s 2 months old.