Let me see if I can help. You clearly care so he's lucky.
His cage is his safe place ,so help to make him feel secure there.
Have a small part of the top covered.so he has a sheltered corner, which to his mind will be safe from predators landing from above.
He will be drinking, they don't drink huge amounts .If you have a tube feeder only, maybe add water in a hopper. A little bath placed on the floor now and then will eventually get used, they like being spruce.
Leave the millet in his cage, don't expect him to come and feed when offered as a hamster might. (Pet shop advice leaves me cross!)That said, too much millet is not good, it's high in cholesterol, so don't always have it available. Do offer lettuce, possibly a little broccoli, spinach, apple. These give essential vitamins and some fluid. Read up on foods to avoid: eg chocolate and avocado are toxic to birds.
Definitely have a mirror and a bell. A little swing will also be welcome. Don't clutter his cage with toys though, give him room to move.
Speak quietly, be gentle ,no sudden movements, and he'll gain confidence. It takes time, lots of it sometimes.
Good for you letting him have some freedom. Leave his cage door open so he can find his own way back to his safe place. I have a little plastic bridge at the cage entrance and let budgies come out by themselves on to that when they are ready, and then it's easier to get them home.
Eventually you can gently persuade them to step onto a perch that you hold and slowly move towards their feet. Say "step up" as you get near and gently nudge them and they do learn to do just that. One of mine now launches herself at any stick I wave if I shout "taxi" as she likes being taken places.
When he is out give him things he can chuck about: little plastic mesh balls, or like mine, paperclips ,pens, office stuff, so the first words he learned were "What are you doing!?" If he learns to speak it will be words said with emotion, so watch out for that. One wee one learned to say "I'm so sorry" as he had a very long car journey home as a rescue and I kept apologising for the constant motion!
There's lots more that will make a difference to you enjoying a happy bird, but I do recommend you get a sensible book ,you could maybe read it with your daughter.
All the best, Perry will have a good time with you.