It can definitely take a lot of patience! When you say he'll take a treat from you, do you mean if you hold it through the bars he'll take it? I've had some very skittish birds and I've found what has worked is just to hold their favourite treat inside the cage - But the key really is consistency. Every single day, and I usually do it twice a day for 5 minutes or so. I just leave my hand in the one spot, never chase them around.
It can feel very frustrating but don't give up

It can take a long time for them to realise 'the hand' isn't a scary monster and if your careful and patient, eventually curiosity or the desire for the food/treat will win out.
This is where I've always started with my very scared birdies and once they're completely comfortable that's when you can move into further handling training.