You are so right, it will take lots of time and patience, they really do take things slowly and being an adult bird he is more set in his ways.
But you are making some progress and small steps are just what you should expect right now, in time he will probably step up for you and sit with you but you have to work up to that for now.
One way to tell about the age is by the eye color, adult greys eyes are more yellow looking versus a lighter shade when younger. The weight of the bird is no indication of age, they vary in weight from 300+ to over 600 grams in weight, some are just bigger. A real indication of whether he is fat is by feeling the keelbone, if it is prominent then he is not fat but if it is hardly felt then he may be too fat.
They do love to mimic other sounds and count yourself lucky that he is only copying finch and canary calls, my Josey screams like my sun conure and that is not really funny.
You are doing a fine job just continue on and in time he will make more improvements.