I wonder why he hated the ex girlfriend? Did she do something to upset him?
Some people believe that greys bond to the opposite sex but this is not true in all cases.
Other members have greys of the same sex and dont have any problems.
A month is still early days,spend time sitting next to his cage and just talking to him, offer him treats and just try to build up some trust.
You have got to remember, even though he may have been neglected that was his home and what he was used to.
The plucking issue, has he plucked since he has been with you? Have you taken him for a check at the vets?
It could have been due to stress or boredom if he had been neglected, get him registered with a good avian vet and maybe give them a ring for some advice on taking him for a check up.
I hope things will work out fine for you both, others will offer more advice and read through all the threads on here that may help you.