I'm so impressed with all the knowledgeable members and the advice they have shared with you!
It does take time to earn trust. But you should be proud of how far you have come already! You will just keep building on that. I've bribed my way into many birds hearts. Birds understand food gifts. Nothing wrong with regard good behavior always.
Birds are often raised in plastic tubs without much exposure or enrichment. That will develop with time and their natural curiosity. When I add stuff I just let them find it and explore on their own. I dont come towards them with new objects.
The advice shared above about providing them places to explore is excellent. Attach perches on the outside sides and top of his cage. Add food and water dishes and easy stuff to chew. This expands and enriches their world. Why not take advantage of the wasted vertical space above the cage? I also use long ceiling hook screws and heavy weight fishing line to hang big rope spiral and swings down to just above the cage. Just above their head height when they stand in too of cage. That way they can grab with their beak and climb up. I hide " Easter eggs " for them to find when they explore , tie on a little millet and such.
Praise , praise when he sticks his head out of the cage, when he starts to explore. I've found Birds to be suckers for praise, flattery, and bragging, even if they don't show it at first. I start off with pairing "good birdie " with everything positive for them . Every time they take a treat, eat, drink, preen, start a nap, play I say " good birdie " to link it with a positive feeling. Soon they understand that, and when you ask them to do something praise and treat. Over time ..maybe a few months or a year you might find they are thrilled to do stuff for just a "good birdie " but I still reward with treats here and there. I still hand out food to them nearly every day, even just a pellet or part of their diet as part if maintenance bond. It makes me laugh to see them leave a bowl of pellets to run to me to take one of the exact same pellet by hand.
Spend time whistling and chatting with him.
Good luck, you are going to get there, and your bond will become more rewarding and deep over time.