Lots and lots of birds don't like to be touched, some like to have their head scratched and nothing else, some like the head, neck, cheeks and under beak and a few don't mind if you touch their entire body (mostly the smaller species but this is something that should not be done anyway). I have coaxed a few of the ones that did not like to be touched to enjoy head scritches but it has never gone beyond that because I don't like to impose on them, I think it's disrespectful (I would not touch a person who doesn't like to be touched so I don't do it to any of my animals which don't like it). My grays are not touchy-feeling birds AT ALL. They like their head scratched, to ride my shoulder, preen my hair, etc but that's it. What I do to get them to accept head scritches is start with 'Piquito' (means little beak in Spanish). When the bird is perching at my eye level, I start talking to it and slowly move my right hand closer while I say: "Piquito, piquito, piquito" over and over. I start by, very gently and very slowly, caressing the top of the beak with the tip of my index finger in a downward motion (start at the lores and down to almost the tip). This allows you to control the situation and not get bit even if the bird is super aggressive because, in order for the bird to grab your finger, it needs to move its head back and look up as parrots, like us, cannot move their top jaw, only the lower one. Once they start accepting this (and they do like it), you start caressing also the sides of the beak by putting your index and thumb together and gently and slowly moving them on the sides (you put your fingers as if you were going to pinch it only you don't put any pressure). Once they let you do this without an issue (they also like this caress), you start putting your fingers a bit higher so, when the movement starts, you are touching where the top beak joins the face. After this is accepted all the time, you can start by sneaking your left hand up and scratch the top of the head with one single finger. Just a touch or two at the beginning because the bird will, most likely, lift his head fast or move. At this point, I stop and ask if they want the 'Pica pica' (means 'itchy, itchy' in Spanish). I put my hand at their eye level with the palm facing down and make a scratching motion with all my fingers - kind of like waving them slowly keeping the nails all in a row. All my birds know what this means and, if they feel like getting a head scratch, immediately lower their head for it - if they don't, it's fine, I don't insist. Even the Senegal from Hell has learned it!
As to trick training, I don't believe in it so I can't help you with that but, if you ask me, he's smart enough to realize they are useless actions with no other purpose than entertain you -and he doesn't feel like obliging you