Thank for the reply, I could not remember if the parents had injured him. That is what it looks like to me. He will have some short little toes but he will be just fine
I appreciate what you are saying about the feedings and learning to feed at the right pace but it is also really important to feed the right amount and get him to the point that he can hold more food so that he can go longer between feedings. It will make a huge difference in his ability to grow. It's not something that has to be perfect today or tomorrow but you should work towards it as a goal. I know keeping records is a pain but it really does help. For instance, in most cases when your bird is growing normally and it's crop is normal size it will eat more food in total during the day with fewer large feedings rather then if you feed small more often. Since your guy is small (does he have a name?) and his crop is emptying within a couple hours he is probably hungry each time, but it is possible to feed too often causing the bird to not eat readily because he doesn't have an appetite. Over the course of a day they will eat far less then if they had eaten several larger meals.
For instance, 5 x 6 ml feedings is 30 mls and 6 x 4 ml feedings is 24 mls. So in this example the baby with the larger crop is getting more food each day, will gain more weight and grow faster. If you keep a baby on a schedule and amount of food that does not increase the size of it's crop it will not grow as well. If your bird weighed 5 grams you could probably be using either schedule but the one that offers him the most food is best.
I only stress this because you asked about helping him gain weight and this is something you can an should be doing. Don't force it but do try and give that extra little bit at each feeding and over the next few days you will find that the crop is slowly getting a little bigger and taking a little longer to empty. It is all part of helping them grow. You can do it.