First of all rule out any medical cause especially constipation.
Increased intake if milk and banana can cause constipation which can also reduce appetite
After excluding that maybe it may be simple food fad or child tantrum or boredom from food.
Try to make food times interesting by letting self feed and play with food.
Make him eat food with different colour and texture.
Don't run to child with food rather look for hunger cues.
No forceful feeding.
Strict timely feeding .
No TV music mobile at feed times.
After doing all the if child us still not eating please consult your local pediatrician who can do a thorough check up to identify any medical issue.