THANK YOU SO MUCH FOR THIS! I really appreciate your trying and taking the time to learn and apply knowledge to improve your birds life! Kudos to you for that!
Birds are all photoperiodic so keeping them at a solar schedule is not something that will make them suffer, that they need to get used to or that they will 'rebel' against - it is what nature ordained for them and the most natural of the all the natural things in the whole world for them. He will get used to it in no time at all - as a matter of fact, I will dare say that, if you do it right, he will do it from day one and there will be no adjustment period at all. This is not any type of boast on my part, mind you - it's that their bodies are made to react to the stimulus so when you expose them to it, the body reacts, nature follows its course and that is all there is to it.
Let me explain why I say this. When the sun begins to peep on the horizon or goes down in the evening, the light hits the atmosphere at an angle, refracting it in a completely different way than it does when it is hitting it straight or almost straight down. This 'slanted' refraction causes the spectrum (the different 'colored' lights that make the white light of the sun) to change so there is less of one type of light and more of another (know how the light seems blue before the sun begins to come out and then turns golden in the morning? or how the sky and everything looks orangey or reddish during the sunset?). It is this different light (which we call twilight) that activates or deactivates their photoreceptors (which they have both in their eyes, like mammals do, BUT ALSO inside their brains because nature figured that light was so important to their wellbeing that gave them extra receptors so, even when their eyes are closed -as when your bird takes a nap during the day- they are still reacting to it through the deep-brain photoreceptors -the light shines into the brain through cranial bones made especially thin so they are translucid - how cool is that?!). And it functions as a stopwatch. The light of dawn turns it on and the light of dusk turns it off - and the body registers the number of hours in between these two daily events to set up their circadian cycle which, in turn, feeds their circannual cycle. The circadian cycle is the one that sets the times for eating, resting, interacting, sleep and all daily activities - the circannual is the one that tells their bodies when it's time to start or stop production of sexual hormones, when it's time to molt, when it's time to migrate, etc. This is why this is called 'photoperiodism' from the Greek word for light -photo- and the 'periods' signifying what we call seasons -as in breeding, molting, resting, migrating seasons. When their bodies are being exposed to the same length of days all year round, year after year, the endocrine system looses track of the seasons and becomes dysfunctional, affecting the whole 'workings' of the body.
All you have to do is look at the sun. Open the blinds, shades, drapes, etc or uncover the cage before the sun comes out in the morning (in USA, this time of the year, it happens at 4:45 am but I stretch it to 5 am because I don't get anywhere near enough sleep during the summer), serve them the heathy food breakfast and, at around 7:30 to 8:45 am, turn on the overhead lights (basically, the sun rays need to be streaking into the room through the window and making the room nice and bright). Turn off the overhead lights when the sun is halfway down to the horizon (in USA, it happens around 6 pm), wait a bit and then give them their protein dinner, and, once it's dark outside (here it happens at around 8 - 8:30 pm), cover the cages, close the drapes, etc.
Now, the trick is for them not to be exposed to any artificial light from the time you turn it off in the afternoon to the time you turn it back on when the sun is already high enough to be bright indoors and, by any light, I mean light from another room, from a street or a garden light, from passing cars, or any other source of light even if it looks very dim to us because there are studies that tell us that even a very soft, very low and dim light is still being registered by their photoreceptors so you need to make sure that the cage cover (if that is what you use) is made out of a blackout material and that it covers the cage completely so no light is sneaking in. For example, the light coming from a TV will prevent the bird from making enough melatonin so it can fall asleep so you need to be very careful about this.
At the beginning, you will find it complicated and difficult to do but, like anything else in life, once you get the knack of it and get used to it, it will become second nature and you will be super aware of the sunrises and sunsets and you will do it as a matter of fact without a problem.