I'm so glad that things with Petie have been going so well!
I do agree that if he'd like to come out, it's just fine to let him out. Some budgies, however, don't immediately leave their cage, as they see it as their "safe space", preferred against the uncertain surroundings. You can open his cage door and see if he would like to explore outside of his cage. If he doesn't, that's fine. Often, it takes time for budgies to work up the courage to peek out. You can put a perch on the outside of his cage like a "deck" so he has a comfy place to sit in close proximity to that which is familiar to him.
If he does decide to come out, just let him explore for a little while. Many budgies will go in on their own after a while, and when a routine is established. For example, letting them out before dinner, budgies will learn to go back in so they can have their food.
Since you already have been bonding with him, there is a good chance he will see you as a "safe spot" and allow you to offer him a hand or finger to ferry him back to his cage when he should return. If not, you can offer a dowel for him to step on, or, if all else fails, just relax and let him return on his own, as he will need to for food/water.
I wouldn't worry about that, though, considering how far you've gotten with him, as well as the fact that you have plenty of millet
Best of luck with Petie's first out of cage time, be sure to let us know how it all goes!