So he gets one 20ml feed at night - does that amount fill his crop? If he's the same size as the alexandrines I've raised, 20ml will not be enough, but he may be a small bird.
If he's accepting solid food- that's a good sign!
Depending on where you live pellet availability may be different - if you have access to the Vetafarm range, try "Nutriblend" pellets (medium or large).
If you have access to Harrison's, try "High potency" (Course or fine).
You can try introducing pellets to him in a small bowl, or if he doesn't seem to want to touch them, soaking them in a little warm apple juice is usually a success!
Chopped up fruit and vegetables such as chillies, apple, orange, pear, passionfruit, spinach, carrot, peas, beans or nuts are great - just make sure not to give him any avocado or onions.
If he likes apple, give him apple! At this stage, the most important thing is to make sure he's eating enough, and getting a wide variety of food so he's not too picky when he's older
The crying he's probably doing right now is due to hunger - it will stop once he gets some food in his belly.