Hi Lauren, and thanks for the tag, DizzyBlue.
It will take time for him to gain new reinforcers. We know seed from his food bowl is a reinforcer - it tastes/feels etc nice. It is reinforcing his behaviour of climbing down to the food bowl or even perching in front of it. Because the seed always comes in the same bowl the sight of the bowl itself is probably becoming reinforcing too. There's an example of a new reinforcer (the bowl) that has been learned by pairing it with an existing reinforcer (seed). So you see how you can build new reinforcers with time.
You've also identified his mirror as a reinforcer.
It is probably reinforcing his sitting in a certain place in his cage. As he gets to know you, your presence will become reinforcing too if you keep pairing yourself with good experiences. Praise might work if he finds your attention reinforcing... it is the animal that chooses the reinforcer. More on praise in a minute.
For now I would just see if you can unhook the bowl and offer it slowly through his cage door - would he eat from it? This would be to slowly desensitise him to the presence of your hand/fingers. It sounds like introducing a finger for the step up at this stage is too much for him. Ditto with the mirror... try unhooking it and hold it up for him first, outside a different part of the cage and see if he approaches it. When he's fine with that, try slowly slipping it through the cage door and see if he will approach it with you holding it.
With both of the above, it's important not to frighten him, i.e. not to go too far too fast. Watch his body language carefully. Any discomfort... eyes widening, feathers perhaps slicking down, leaning away... back off a little until you find a distance that he is ok with. Then come forward again a few millimetres... reinforce his calm body language each time with backing off again. Slowly slowly over days or weeks you will be able to get close enough for him to eat/touch the mirror. Keep these sessions really short... 10 - 30 seconds here and there through the day, and be sure always to end a session on a successful step.
When he shows the calm body language through the desensitisation process, as well as backing off slightly each time, you can also use your voice to praise him... you are pairing your voice with something desirable so that your voice/the words you use will also become reinforcing in time. (Just like the food bowl being paired with the seed.)
He may be totally fine about eating from the bowl with you holding it or embracing his reflection in the mirror again with you holding it so I'll wait from some feedback from you before continuing.
I would continue clipping millet spray inside his cage because he might try it eventually... he's probably never seen such a thing before. You could try cutting one in half and clipping it to the side of his cage... that way it won't move around so much which might be a little frightening. Keep trying with the veggies and fruit because again, one day he will try them. It took my Kobe almost 2 years before he would try raw carrot.... that was a lot of wasted carrot, but we got there in the end. Likewise keep trying with toys. Even if he doesn't touch them to start with. Have a few different ones that you can rotate in to keep up the novelty. You want to try enriching his life as much as possible... and yes, it will take time with a bird that doesn't know how to play. My Ollie didn't know how to play so I started mainly with tiny toys tied to the side of the cage so they didn't move around to much. Nowadays he doesn't stop playing... so playing can be learned in time. We know your little boy likes mirrors... perhaps build on that... get a toy that has a mirror but also other stuff on it. My budgie Boo's favourite toy was a plastic "wicker" ball that had a bell inside and I attached another bell on the bottom and hung it off a small chain.