Daylight Savings Bedtime Difficulty

My gf’s son is almost 6 years old, Level 2 autism. Bedtime is often a struggle already, but now that summertime is here, it’s so much harder for him (and us). He doesn’t understand the concept of time, and is frustrated when he’s told it’s bedtime, because “it’s still morning out!”.. the sun doesn’t set until nearly 10pm. He has a whole bedtime routine (movie in bed for about an hour or so, and then my girlfriend goes in to lay with him. Movie still going on mute, and they lay together until he eventually passes out).. this usually goes okay, but now it’s so much harder to get the routine started.. and he frequently doesn’t fall asleep until around 11pm. Have any of you dealt with this struggle, and is so, how did you conquer it? Only thing I can think of is complete blackout curtains, and making a social story or something, to explain time a little bit, and that sometimes the sun stays out longer, but bedtime is still at ___pm, no matter what.

Just curious to hear your thoughts & any ideas! Thank you!!