When we were hand feeding Makena we would trade off his formula feeds with a pureed papaya, pine nut, pea and banana blend. We would usually give him formula in the morning and at night and give him the pureed blend for lunch. He seemed to really enjoy it, plus I think it helped get him used to other foods.
We got him at 10 weeks old (which was WAY too early), the breeder lied to us and said he was weaned. They never should have sold us an unweaned bird, no one should be. They also lied about his age, they said he was 4 months old...only to find out later he was only 10 weeks. Unfortunately there was no way of taking him back to the breeder as we had to fly to get him. We also found out that Makena had some problems (worms and lice) so we didn't want to take him back to the breeder and have him exposed to that again. Lets just say we will never go back to those people!!!
So basically we had to be learn to be good handfeeders quick. We did lots of research and were very cafeful. Lots of things can go wrong. Luckily I am a food scientist and have lots of equipment at home that came in handy (digital thermometers, digital gram scales, drying racks, etc). We were very careful to sanitize after every use and feed at the right temperature (Makena liked it between 105 and 108 degrees). Also make sure to heat the water, then add it to the powder, do not microwave the powder as there can be hot spots.
We fed him through a syringe until the very end, he was never a fan of the spoon. He would eat around 45-55ml of food at each feeding. We gave him 3 feedings a day up until 4 and a 1/2 months old, then one week of 2 feedings and one week of one feeding. It was exactly 5 months before he was fully weaned. Obviously this is different for every bird, but just something to expect. We never force fed him, we learned to read his signs and would feed him when he was hungry.
So there is some info on our experience... hope that helps. Definitely take him back if he still isn't eating... this is a very critical time for the bird, he needs to be getting sufficient calories. Good luck!