Thank you for not losing hope. Parrots (especially African Greys) tend to move in slow motion. Much like a child with a serious trauma history, these birds can be very slow to trust and fearful.
It seriously can take a long time---I "rescued" an umbrella cockatoo (I am her 4th owner) and, comparatively, she didn't have "much" trauma (aside from repeated re-homing and possible abuse in her 2nd home).. Either way, her last placement was long enough that she seemed pretty well-adjusted until she was uprooted again. She is currently 11.
That having been said, it took 3+ months for me to be able to hold her...and cockatoos are way more physical/needy the many Greys. Things are great now, but it was slow moving.
My uncle had a wild-caught TAG when we were growing up (he bought the bird in the late 70s so things were sketchy) and that bird was family, but for the "kids", he was the kind of family that you do not touch unless you have had a few glasses of wine haha. His TAG was GREAT with him after about a year or so, but there were some over-bonding issues.
I am rambling, but the point is, you must be very consistent and very patient.
In the meantime, make sure that he is getting a solid diet, at least 12 hours of sleep nightly, and avoid allowing him access to any shadowy spaces (hammocks, low ledges, boxes, hollows, pillow piles, under furniture etc). Hormonal behavior can be triggered when birds are allowed to interact with shadowy areas and hormones can lead to heightened cage aggression, plucking, screaming and aggression in general.
Additionally, try to keep his light cycle/bedtime routine consistent---light and sleep regulate hormones, so you will want him to wake up and sleep at the same time (roughly) each night.
Make sure you are not using any scented or chemical products around the bird, and keep an eye on humidity levels. You do not want to use any irritants around the bird (perfumes, household cleaners, lotions, nicotine, hairspray, air fresheners etc).
If you can do so safely, try to allow your bird out even if you cannot touch him---if you can wait it out, he will eventually go back in. The more time a bird is locked up, the more likely it is that they will become cage-bound and paranoid.
Try to take it slow and keep things safe and predictable. Also, work to associate yourself with as many positive/low-stress activities as possible. Work on building trust and try not to push physical contact.
If you have not done so already, at some point you should also get a cbc (blood test) done. Birds should have one yearly.
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