It's really hard to break the habit, when dd1 was like that with a newborn on the way I resorted to thermal Sippy cups with milk that she could find in her cot and have while we sorted out her daytime foods. But she was happy with cold milk. Make sure he is getting iron as it stimulates their appetite and find foods he likes and let him graze all day and encourage more at night/dinner time doesn't really matter what food it is for now you can slowly swap it over. As a last resort add in more milk during the day so he feels full instead of wanting it at night. We finally swapped the milk at night to water when dd was 3.5.