It does take a while for a full set of flight feathers to all come in as they are usually replaced in stages.
Since he has been in several former homes it is possible he is hand shy and doesn't want to perch, rehomed greys carry a lot of baggage with them that they collected from their previous homes and it is hard to work thru that but time and patience will be your best friends.
I do give my grey some red palm oil every day, you can add it to other foods or drizzle it over pellets or his seed mix, I stir a little in some oatmeal that my grey eats from a spoon, she gobbles it right up.